I don't have Steam Deck but I have experience with UConnect, and it's easily the worst 3rd party launcher out of the others. Constantly saying that I have no internet connection despite it working very well, I can't even launch games because of it.
I have the South Park bundle from Steam. The Ubisoft launcher wasn’t an issue, more of a minor annoyance because you need to log into it every so often.
A bigger issue is that both Stick of Truth and Fractured But Whole suffer from random freezes that require killing the game from the Steam Deck menu. It doesn’t appear to be universal, but other folks have said the same thing on ProtonDB. Tinkering with proton versions hasn’t fixed it for me yet.
Ubi connect will not work offline, even if you login online once and then immediately take the deck into offline mode. Connect fails to launch the game.
If you get all the way into the game then go offline I believe it still works. I’ll have to test again but I do recall being able to play without internet after resuming from sleep
For me it depends on which one. I installed a game yesterday which used the old launcher (uPlay). Whatever I did, no matter which version of Proton, the launcher would not connect to the internet. It offered me offline mode, but because it hadn't yet connected once, that's useless.
The solution is a bit strange. I went into the prefix of this game and overwrote the ubi launcher folder with one from a prefix on a different game -- crucially this one is a later version (Ubisoft Connect), even though the launcher folders have the same name. I let it overwrite any files with the same name.
Started the trouble game again, and Ubisoft Connect (the new version) came up. No issues connecting to the internet. Was able to play without issue.