I remember it being surprisingly decent for games back then. Then a lot of games moved to Steam, which made it way harder to run them in Wine. Of course there was later Proton for that, but not on Mac.
Games are one of the easier things to emulate since gaming mechanics are usually entirely a compute problem (and thus not super reliant on kernel APIs / system libraries). Most games contain the logic for their entire world and their UI. The main interface is via graphics APIs, which are better standardized and described, since they are attempting to expose GPU features.
I worked on many improvements to wine's Direct3d layers over a decade ago... it's shockingly "simple" to understand what's happening -- it's mostly a direct translation.
