I believe my main form to be exactly that, RDT DSX with basically a zero degree slant of the pick, and I definitely cannot do continuous USX, I can however do it as a helper motion from my wrist, but I try to avoid it or minimize it when playing fast, since it takes away from my speed noticeably, it’s still fast so it’s usable for me as a helper, but not nearly as fast as pure DSX, or like the guys that can do DBX or secondary motion at will reliably like MAB or Kiko Loureiro. But if my memory serves me well zero degree pickslant should allow for DBX.
I’ve noticed a lot of guys do it Gilbert, Tremonti, despite being DSX main, but their pickslant looks neutral.
But Mike Stern from the looks of it has a slight downward pickslant. So I guess for continuous USX you do need it even a little bit.