It'll have some small issues, of course. From what I can read online some plugins don't work because they rely on the Windows Script Host (the thing that runs JS and VBS scripts) and there's a performance issue here and there, but that's about it.
The thing about games is that there's only a limited OS surface area that they'll usually touch. There's I/O, GPU, a single render Windows, audio, and input state, but when those core APIs work, games should run just fine. Boring Win32 software can get real dependent on some obscure system APIs, COM+ objects with certain properties, library loading behaviour, etc., all things that can be a challenge to emulate successfully without tripping up programs that assume certain APIs just never fail or return certain results. Games don't call APIs like DsRoleGetPrimaryDomainInformation or SHEnumerateUnreadMailAccountsW so projects like Proton don't usually add a lot of fixes in that space.
The thing about games is that there's only a limited OS surface area that they'll usually touch. There's I/O, GPU, a single render Windows, audio, and input state, but when those core APIs work, games should run just fine. Boring Win32 software can get real dependent on some obscure system APIs, COM+ objects with certain properties, library loading behaviour, etc., all things that can be a challenge to emulate successfully without tripping up programs that assume certain APIs just never fail or return certain results. Games don't call APIs like DsRoleGetPrimaryDomainInformation or SHEnumerateUnreadMailAccountsW so projects like Proton don't usually add a lot of fixes in that space.