I think synchronization depends massively on our ability to create and associate temporal points of reference for our movements. The more of these points of reference we can create, and the clearer they are, the easier synchronization will be.
If our reference points are insufficiently many or clear, the coordination becomes “fuzzy”. It’s difficult to synchronize fuzzy actions to each other or to an external tempo.
To me, the defining characteristic of movements which allow for the creation and association of these reference points is impact. For the picking hand, we have the moment of impact of the pick hitting the string. For the fretting hand, we have the moment of impact of the hammering action of the finger while fretting, finger to string to fret.
When we look at the picking hand, most players are able to keep a strictly alternating movement on a single string in time with an external click. The sensation on impact of each pickstroke creates a tactile reference to a point in time.
However, sweeping is notoriously difficult to play with good synchronization. Instead of clearly felt individual impacts drawing our points of reference, the pickstrokes “blur” into eachother and we lose our point of reference for each individual note. Instead, we are forced to synchronize based on the endpoints of the sweeps, where we change our direction and feel that clear moment of impact again. Banjo rolls also have this “blur.”
We experience blur for the fretting hand also. There is no impact in lifting a finger off the fretboard to reveal a lower note, or in a pull-off, a finger roll or a slide.
Percussive actions are easily synchronized, but factors can result in even percussive action feeling woolly, making it dificult to distinguish the moment of impact. A very soft pick will flex and absorb impact when picking. High action separates the moment of the finger hammering the string from the moment of the string hammering to the fret.
The moral then is that impact is your friend. Create as many clear impacts in your guitar playing as you possible can. I know that for me personally, my fretting hand became much more esily to synchronize either to my picking hand or a click when I learned how to play everything with hammers (ascending, descending and from nowhere).
Also, something my father (a very good drummer) told me has always stuck with me:
If you want to groove, you have to move.
Groove is a body thing. Your ability to internalize the pulse, your sense of time and rhythm, your ability to follow a groove and find your pocket are all contingent upon your movement of your body. Whether it’s tapping you foot, bobbing your head (like every bass player ever) or anything else, allowing yourself to move will help you to form that connection between your body and time.
For experienced players, the movement of your picking hand or your fretting fingers can be enough, but in the beginning when we’re learning unfamiliar movements, creating another temporal reference with our bodies is massively helpful.