Over the weekend whilst away I played wonder boy the dragons trap on my steam deck, absolute perfection on frame pacing, the OSD monitor was flatlined for frame deviation.
I then loaded it up on PC and when I enabled modern graphics it was stuttering, luckily I was able to fix it easily by disabling the in game frame rate limiter, and vsync kept it at a perfect 60FPS. but interesting that quirk didnt happen on the steam deck.
Before I left I downloaded about 20 games or so to the deck, and it would have been about 25 or so, but some were labelled as not compatible, so the impression I get is the majority of games are playable. Although it has to download so many dependencies. Specific versions of proton libraries and so on. It is handled automatically though.