Just finished my first mission on the Steam Deck, and I'm kind of shocked that it worked.
Anti cheat is NOT broken on SteamOS, contrary to speculation I've seen. You can absolutely play this game online.
Performance in game is mixed. I averaged around 27fps using the Performance resolution scale, and medium->high graphics settings. Not great, but serviceable.
At low settings with native res, I get around 38fps
At low settings with ultra performance scaling, Im getting 60fps, but it looks like a Switch game. Smoke shaders look insanely pixelated, and foliage is super jaggy.
Unfortunately I encountered general weirdness out of gameplay. The first run took a couple minutes to get to an in-game screen. I was beginning to think it was broken, but eventually it did start. Not sure if that was an anti cheat initialization thing, or a shader compilation or what. Subsequent launches are normal. I think there's a few bugs in the game that show up more obviously on a low power CPU. Changing the resolution scale will reliably make the game hang.
Overall this is pretty impressive. I wasn't expecting it to work at all. When Valve gets around to rating it, they'll probably say Playable. If Arrowhead does an optimized Deck mode and fixes some of the general issues, I could see it get Verified.