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 (I don’t think it blocks DNS) but good enough for what I needed.