I would like to ask is it legal to use Steam through wrappers like CrossOver?
I currently have an account which current assets I have gained by years of hard work, so I think that is reasonable to ask are there any warrant of not getting banned because of the emulation? It is so nice to see all of my applications and loved Windows-only games could work without any hassle, I am just a bit unsure...
Legally, Crossover/Wine is just another Win32 implementation.
It is not against the Steam EULA. There is nothing against alternate Win32/Win64 implementations, porting stacks, or emulation stacks in there at all.
Several game engine companies have stated that the Wine stack used in crossover is not against their EULA. Unity3D's statement on the Wine stack used in Crossover is that their clause against porting and emulation stacks are referring to mobile devices and consoles. Mac and Linux are considered Desktop platforms under the EULA and thus it is not a porting stack or emulation stack, just a compatibility stack.
As of anti-cheat:
The Wine API used by Crossover has never set of VAC systems at all. VAC uses a combination of variable out-of-range detection, and module blacklist detection. The API does not do anything against that.
VAC is very good at not false alarming, and has never been known to false-alarm on the Wine API used by Crossover.
Most other anti-cheat systems will only ever fail at the initialization stage where they don't result in bans. Under Steam's standards you cannot receive a VAC ban for all games for other anti-cheat systems.
Still, I would test that the game in question works on your machine with the anti-cheat system enabled before going and playing online with any game that has anti-cheat, just to be safe. (Generally there is a practice mode or tutorial you can do this in, if there isn't a single player campaign or local play against bots). This is also just about being polite. Make sure things are working before you play online because you don't want to bail on the other players if they are not.
We're rebels. We're misfits. But mostly, we're software liberators. And we're very, very good at what we do. We have to be. Lots of developers work with open source, but only a tiny fraction of those are good enough to get software that was designed for one platform to work on another one. We invented CrossOver software - a unique approach to cross-platform compatibility that does not require dual-boot or another OS license. We launched PortJump to help app and game developers broaden their market beyond Windows® users. And we launched ExecMode to help organizations solve really ugly technical challenges.