Just chiming in as someone who's relatively new to Linux gaming for anyone curious or on the fence. In the 4 months or so of being on Arch Linux, I have encountered zero games that don't run despite playing a large variety of games.
I'm not saying they don't exist, and I'm not saying there aren't hiccups or bugs out there, but boy is it a lot closer to "completely seamless" than I think most people imagine.