So, my personal experience of the installation is been a bit complicated by the tutorials i've find on YT. Basically I was stuck in the part where after the installation you need to change the directory (or add a new shortcut) for the launcher. Many video didn't explain that or use a previous version of the OS with slightly different interface. In the end i've found this great tutorial online and completed everything.
There is anything i should do (both on the device setting or in the Ubisoft setting) to improve the experience on my Steam Deck?
Btw. The download of the game (AC Origin) was superslow (9mb/s). Any way to fix that?
Lutris has many functions, but what I use it for is a non-steam game Wine/Proton manager.
Basic principle of how Steam runs Windows games on Linux. Steam uses a modified version of Wine, called Proton to convert the native windows code to something Linux understands and can run. And has a pre-made Wine environment with all the appropriate files and directories so that when the game installs, it thinks it's in Windows.
Now Lutris does pretty much the same thing, just without Steam. It too has pre-made Wine environments ready to install games, then with what ever version of Wine or Proton you have installed, you can install the game. For Ubisoft Connect, you aren't just installing a game to a Wine Environment, you are installing a software manager to a Wine Environment, which can install other games, require software to your Environment, like on Windows.
Another benefit of Lutris is that if a game requires a specific dependency or needs to run with specific files and stuff, there are pre-made installers that will prepare the environment for you. For example here is the Ubisoft Connect install script: https://lutris.net/games/ubisoft-connect/
Plus since each game is installed to its own environment (except Game Launchers for DRM reasons), it means you can remove a bad environment without deleting all your games. This is a real boon if you buy your games from GOG.
However for the process it's rather straight forward.
Find the game you want to install on lutris.net, if it's not there just download the installer and try to run it.
Let Lutris install the program
Try to run the game in Lutris
3a) If it works, right click the game in Lutris and "Add to Steam"
TL:DR Lutris installs Windows games like Steam, and runs them like steam. But it doesn't limit you to Steam games. Ubisoft Connect, EA App, GOG all work on Lutris to varying degrees of success.