guilty gear XX is a VERY technical game,
not deep in a way like 3S where u gotta play a lot of mind games
but technical in a way like:
ok, I jus landed a hit on my opponent, how can I take advantage of this to continue hitting him
or
ok, Im getting hit, given a chance to escape the attack when would be the best and safest time
during comboes when you're gonna do things like jump installs, jump cancels, and false roman cancels, controllers and especially a PSP unit would not allow the freedom your hands would give holding a joystick instead.
furthermore, using other moves such as insta air dashes and dash canceling would be sloppy as well.
finally, some characters are jus straight up impossible to play without a joystick. perfect example, Eddie
his game requires lots of holding down buttons while comboing with other buttons so that baby eddie wont unecessarily transform.
unless you hold the controller very awkwardly, this can only be done with a single finger on each button.
but seriously tho, I havent even played this game in like 2 years...