I was able to install Elden Ring and play without any changes. It carried over my save from my PC. It just worked. Same with Vampire Survivors. I use these two

I was able to install Elden Ring and play without any changes. It carried over my save from my PC. It just worked. Same with Vampire Survivors. I use these two as examples because they are opposites of one another in terms of resources. If ER and VS can both provide games that work out of the box with the Steam Deck, then it really falls on the games developer, and really, do they support controllers? Steam makes it clear what works well with the Steam Deck. Not everything does for various reasons, but if you stay with what works well, my experience is you'll be fine.