But... Enemy Starfighter/House of the Dying sun has arbitrary dinput HID support. I can play it by using an F310 in xinput mode, Logitech racing pedals in dinput, and a keyboard simultaneously if I want to.
"full" stick support is tricky. If you want a ton of joysticks, HOTASes, rudder pedals, and other sim **** to be supported out of the box with keys all mapped correctly without the user having to bind everything manually you'll need a lot of work.
But for anything else you just need a dinput wrapper to translate the (up to) 8 axes and (up to) 128 buttons to controls your game uses. Even if the controls only show up as "BTN 1", "BTN74", "BTN5", etc."
Without this ability tools like vjoy and UCR would be useless since the virtual sticks it creates can have weird and impossible(for a physical stick) layouts.
Supporting xinput pads is much "simpler" because they're all guaranteed to have the same layout so you only need a single control scheme for all of them. Basically, xinput makes "plug n play" extremely easy, while doing the same with any arbitrary dinput HID is nearly impossible. But if you just want basic functionality and let users deal with the rebinding then dinput can work as "one size fits all".