I recently got a Steam Deck and, of course, one of the first games I installed was Sea of Thieves! Sure, it's no match for my gaming PC but I'm very surprised at how well it runs for a handheld device.
By default it applies the "Rare" graphic settings and I was getting around 45 FPS. I decided to cap it at 30FPS though to save battery and give a more consistent experience.
Still, having a game running on a handheld that looks and performs better than a Day One Xbox One is very impressive. Unfortunately the ping isn't great, it's double what I'm used to and very likely down to the WiFi and using a 4G phone hotspot. It hovers around 120-140ms, which is still playable (EDIT: depending on WiFi signal and how good of a 4G connection there is, I am seeing pings as low as 80-90ms, which is a lot more playable!)
The controls are okay, though a little fiddly if you are used to using a proper controller. But that's more a review of the system than the game itself. Another limitation of the Steam Deck is that it doesn't detect the difference between 3-pole and 4-pole 3.5mm audio jacks - what this means is that if you plug in a standard headphones without a mic, you can't use the onboard microphone. You can still use the onboard mic if you use Bluetooth headphones though.
I won't be getting into PvP anytime soon on it but it's a nice option to have if I'm stuck in work, or away from home, and have time to kill that I can just pick it up and do some fishing or chill activities for a bit (got those milestones to chip away at now!).
I don't know if Rare have any direct involvement in the Proton compatibility but whoever has been working on it has done a really good job.
Update September 2022: There seems to be an issue with audio support on the latest Proton versions. If you experience a drop out of audio when you log into the game on Deck, then force the game to run under an older version of Proton. See my comment below detailing how to do this, if you are unsure.