I have homework. 
I will say that the existing DSX material when I joined, I thought the “vocabulary” of available licks was much more interesting in the USX stuff, partly because McLacughlin just phrased and played in a way VERY different from me.
Some of this may be as simple as USX just seems far more prevalent in rock, and I wonder if part of that is we’ve all been told at some point to do 1-2-3-4 chromatic drills, which if you start with a downstroke is all USX.
The (perhaps) natural tendency to associate accents with downstrokes might have something to do with that.
Honestly, this has been something I’ve been working on lately - I tend to want to start all phrases with a downstroke, because, well, when practicing I almost always start on a downstroke. but, there’s something to be said for getting into the pocket by starting on upstrokes if you’re starting on an off-beat, so your downstrokes do then fall on downbeats.
Doing those 1-2-3-4 chromatic drills starting on upstrokes feels weird, but is getting easier and should flow way better for a DSX player, so - to my wife’s delight, I’m sure - I’ve spent a lot of time on the basement couch tapping a beat with my foot and playing chromatics. 
Two examples!
The tutorials show lots of examples of non-spiky USX motions. A camera is the best way to know if you’re doing any of them. When you view yourself in slow motion, the pick should be moving diagonally in an obvious way. You don’t need a Magnet if you don’t have one. Just use a cheap tripod.
For garage spikes: On the platform we have a TC going right now with a nice loop of the player’s own clip showing the issue. If you want to see what a real-world example of USX garage spikes looks like, this is a great case study. You would need a membership to view the details. But it’s the one called “Confused Grip,” and you can find it in the TC directory on your dashboard.
More generally:
I would suspend judgment about whether you have a garage spikes problem until you can actually see it. Filming is the simplest way to know. Garage spikes has a very specific appearance: one direction of the pick will move across the string with no drama, and the other will dig under and get stuck.
Even if it is happening, the fix is still to do any known good USX technique as per the tutorials. For general USX playing it doesn’t matter which kind. Anything that goes fast, feels easy, has good attack, and has an escape will do.
This kind of “repeat exercise-type phrases to a click or tap” practice is something I get nervous about. It’s very easy to gaslight yourself that your technique is changing when it’s not.
If the thing you’re working on is something specifically memorization or timing oriented, maybe, sure. But even then, for things like hand sync, I just use the internal metronome.
For the recent Joe Pass thing, it was very much a “memorize the unfamiliar solo” task. But I did not use a click for this. Somehow I just find it intrusive and distracting. Like it’s yelling at me to stay constant when I don’t even know what comes next yet. I can do click once it’s nearing completion and I want to “play along with Joe”. I did that toward the end to make sure I could get through the whole thing for the split-screen playthrough. But not at first.
Once more banging my “Vogg starts inside Gilberts on an upstroke on the upbeat” drum lmao
Flip side: Learning to be able to start phrases on an upstroke (say, descending Gilbert sixes) was huge for my consistency and sync.
This is not intentional, and may be somewhat just appearances. The seminars are two and two: Antigravity is all DSX + 2wps and Metronomic Rock as you point out is all DSX. We added it for precisely that reason, i.e. balance.
The instructional approach in the Primer biases DSX and has for a while. We have taken great pains to repeat how common the Al Di Meola / Andy motion is, since so many players show up doing it:
So the reverse dart sequence starts with vertical pick + DSX or DBX, assuming that most people will land in this bucket – and they do. We have one USX lesson as an adjunct after that.
I don’t notice an overly “USX focused” mindset in the TC players. They seem to get the idea the best technique, at least in the short term, is the one that actually works. So I think the message is getting across.
But we’ll keep an eye on this moving foward and make sure that the “good is good, regardless of style” mindset is reflected in the teaching.
I think of this more like “the single note is the downbeat”, and I sometimes just start there. I think this works because the 2wps motion lines up with the chunk, so everything repeats on the beat. Edit: sorry, for the “inside picking” version. It’s very tidy that way. Downstroke, downbeat, 2wps motion.
Yes, this works for any way you want to play it! There are just some particular musical applications where I want to not have to start on the downbeat. Not really a huge change tbh.
Full disclosure - for me, it’s twofold - one half IS getting more comfortable on starting phrases/hitting the downbeat on upstrokes.
The other, is my internal metronome is not that good. I tend to do a lot of free time playing where if timing drifts a little it’s not so obvious, and I’ve found that tapping my foot or bobbing my head in time ot what I’m playing really helps me get into the pocket better. So, by tapping my foot to the 1 while I’m playing, I’ve found my playing becomes significantly tighter. This is at playing well below my theoretical ceiling and very much is about getting into the pocket.
(It’s actually something I didn’t realize how bad my internal metronome and sense of groove had gotten - and, even noting that tapping your foot is hardly a rigid beat, it’s at least an internally consistent one and makes you “feel” what’s a downbeat and what’s an upbeat a little more)
This is another thing I’d film. In our interview, Frank Gambale does obvious two-way pickslanting for attack reasons, often with finger / pick grip adjustments, during two-way sweeping. I’m sure there is a timing element to this, in terms of making sure the orientation and motion match up. If a phrase feels weird, I would expect there to be something obvious and visual happening there that might help understand why.
Good idea, I’ll do that. I’ll send it in for a technique critique if I’m having trouble seeing the issue!
Also a lot of hard rock and classic metal riffing (especially 80s style) features a lot of steady 16th note rhythms. Think the kind of riffs Randy Rhoads wrote in Ozzy. I would imagine if that was your intent to play riffs like that, that you might develop a USX motion.
Do you find that USX favors players who use forearm rotation? I find my DSX style compatible with my wrist deviation (or RDT if that is a wrist mechanic) movement but when I try USX I find myself having an easier time doing the upstrokes with forearm rotation.
I agree with this estimation in the sense that I can only describe how things look like to me as an individual but must acknowledge the fact that the DSX content is there and its mechanics and benefits are properly described in the educational material. Perhaps it doesn’t feel as flashy as “this is how EJ plays pentatonic fives” but again, that’s completely subjective from the perspective of the viewer.
Not starting on the downbeat feels to me like an easy enough strategy to improve your pocket and alleviate some of the harmonic pressure implied in landing on a strong part of a bar/beat.
Soloing becomes more fluid and easier to manage over extender periods imho because it kind of feels like you can start and end your lines in any way you want, which might result in something more “oh Steve ended the lick on a major 9th” and less “Steve missed the chord tone”.
I would have expected this problem to be extremely rare. It would seem to me that:
What am I missing?
Me.
I covered all the variables you mentioned exactly the way you described them and still experienced garage spikes. I don’t think they align with the root of the problem.
In my case, the problem seemed to be that I took the concepts of down and upstroke quite literally: downstroke to my feet and upstroke to my face, and it took 2 CTC experts to help me realize that a different path could be more beneficial: downstroke to my right hip, upstroke to my left shoulder. That diagonal path is gold imho.
Jason Becker used a lot of index and thumb articulation. You can see multiple different thumb index actions on this video
When it says improvised solo, you can see the thumb moving a load. That at the 1 min mark
Some solid non moving, some moving the index and thumb a lot, gliding over the garage spikes.
Sorry it’s a 40 min video but it’s in there, my pc died and hard for me to get time stamps, if you look at his right hand for the alternate picking, there is large index and thumb articulation.
At one min etc you can see, a lot of up down adjustments of the pick in the index and thumb.
Not saying this is an answer for garage spikes, but I have seen him do this without changing pick attack much, pick slant etc.
I think the action on your index and thumb massively depends on you hand size. If you got big hands your smallest movements do a lot more than if you’ve smaller hands. The size of you hands can let you do things others can’t.
I’m delighted that you figured it out.
Note that the CtC material carefully avoids thinking about the guitar in the body’s coordinate system whenever possible — everything is usually relative to the face of the guitar, or otherwise people would need a photo of the player. For example, if I was fat and had the guitar resting somewhat on my belly, what you described would be strong USX for me; it’s impossible to tell without a photo.
Glad you were able to figure that out! I just want to point out that the Pickslanting Primer has had a lesson on this very subject for years now:
Note that this is not magic, and not a general “rule” about how picking motions are supposed to work. The path the pick follows is entirely dependent on which technique you are trying to do. For example, Joscho Stephan’s technique really does move more aligned with absolute up and down. But this is just due to his posture, the wrist motion he is using, and so on.