I played it a bit. Not enough to call for whitelisting, but I killed a few things and used a controller to do it and found zero issues along the way. Dungeon Siege III is one of the games I've encountered that just works. That test was in Proton 3.7.3 rather than the beta.
system: https://gist.github.com/nstgc/02457d7920931a3f8d69cc7b9fa41a2c
Works fine for me. My specs - https://gist.github.com/GriefNorth/7f4820f319fda7e45ff44c5b83c20cd0
This game has a game breaking bug for me - the mouse is only able to move in a rectangular box to the left, not covering the whole screen on proton 4.11-7.
proton 4.11-7x1 2019-10proton 3.7x1 2018-08proton 3.7-4x1 2018-08
Loved this game, runs perfectly without performance penalty (that I can see) on highest settings.
System Info: https://gist.github.com/stratus-ss/73e90ae7dcc42caf324190ea36e979cc
Proton 3.7-4