Baseline (Widescreen) Solution & Issues
Native. Resolution can be changed at any time from the Options, Graphics menu. Game is anamorphic 16:9 in 16:10. Fix is available to remove black bars in 16:10. It can be downloaded from PC Gaming Wiki
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
The game accepts the resolution and only the cut-scenes and menus are showed with lateral black bars.
Black Bars fix for cut-scenes: Replace ACBSP.exe in the game folder from this archive - Brotherhood_21_9_BlackBars_FIX.7z which is available on PC Gaming Wiki
Eyefinity / Surround Specific Solution & Issues
The game is Anamorphic in single screen, but is Natively Hor+ for multi-monitor. A few playable sequences (QTE in some cut-scenes and the intro to each Truth cluster) are pillarboxed. However unlike in AC2, they are little more than cut-scenes really: you only have to move forward or click a button, no jumping/climbing involved.
Although resolution support is the same, the MP game is a bit different: framerate is capped at 60 fps and screen change is anamorphic in multi-mon.
Black Bars fix for cut-scenes: Replace ACBSP.exe in the game folder from this archive - Brotherhood_21_9_BlackBars_FIX.7z which is available on PC Gaming Wiki
4k UHD Specific Solution & Issues
Stereoscopic-3D Solution & Issues