I had the exact opposite experience - I tried out about half a dozen games, all of them worked and ran well. But of course, it's going to be a subjective experience, especially since (as you said) there are some games that just straight up don't emulate well even on a powerful PC. MGS4 runs at a poor framerate whether you play it on a PS3 or emulate it on a Steam Deck or PC - but the fact that it runs the same on PS3 and Deck is a testament to both the Deck's power and how impressive emulation has become.

Meanwhile there was a guy at r/GamingLeaksAndRumours trying to convince me that the half a dozen games I tried out and worked just so happened to be the only PS3 games that emulate well on the Deck. Which was really funny.