That only works on the 2 or 3 buttons that are hard-coded to support it and doesn't work in the general case (it can't work in the general case because it's lit

That only works on the 2 or 3 buttons that are hard-coded to support it and doesn't work in the general case (it can't work in the general case because it's literally just a long press, which already does other things). Personally, pushing on the flashlight or camera buttons with 3D touch actually feels really satisfying, because it really does feel like I'm clicking a button even though I'm just pushing on my screen.