It causes a bunch of weird controller errors, such as when a controller disconnects (or if the deck goes to sleep), it can't reconnect again.
Just let it use the default version (which I believe should be Proton Hotfix, or whatever Valve decides it should be since BG3 is an AAA headliner, they'll want this to work)
The game is fun, but I would definitely not say that it runs great. It's runs at a bare minimum 30fps with low/medium settings and dips below 30. The FSR implementation is awful and basically unusable - it blurs your character beyond recognition. I have to use the Deck's frame limiter and FSR to get it to run decently. The game is not well optimized.
But as I said, the game is cool. Worth a play if you like rpgs.