In my experience, it can handle most games that expect the Steam client/libraries to be present, so long as DRM is not involved. Some games might require special configuration, like generating an interfaces file, which is documented. So… pretty reliable?

I have also used the experimental build to block internet access for a game that was trying to collect data from my system and phone home, without breaking LAN multiplayer features. Not foolproof, but good enough for what I needed.