The main thing is that concave buttons aren’t flat, so unless you’re 100% used to them, they will be annoying for your fingers. This is especially true when you just react to something and press a button (like tech throw, pressing jab, etc). Lots of times you’ll press the edge of the button instead of the center. While you probably won’t have any problems with this, it can eventually make your fingertips tired. Especially if you’re placing in the arcade and one of the buttons is shoddy.
I vote convex because a) they are flat, meaning anywhere you press feels natural, b) they are much more comfortable when mashing (more of a marvel thing), but also good for supers in cvs2/st etc, and c) they have a more even feel when you press them. I don’t know why they feel more even to press, but the spring feel is really strange with concave buttons.