The biggest steam deck compatibility issue is that the multiplayer mode wasn’t supported. From what I understand that hasn’t changed, but they moved it to a separate free DLC so it won’t affect the steam deck rating.
Other related improvements made are UI changes for steam deck, a deck graphics preset, updated FSR support, and other performance improvements.
Valve runs multiple gamble services i.e. virtual item markets. Their hardware uses the same ressources from very questionable sources as any other hardware vendor. They may be customer friendly, but they are not ethical.