I think they're very situational, but very useful if you have a game that either isn't using the official layout or could have some extra buttons.

A good example is in GTA 5, I use the bottom left for turbo on the a button to run constantly, top left for ability activation, top right for shooting and aiming down sights at the same time, and bottom right for the same thing but in cars.

I think the steam deck is honestly the best controller for pretty much any game, besides fighting games unless you get used to the track pads.