The issue is not the launcher. The issue is the exclusivity. It demonstrates an anti-consumer mindset. The GOG, which people here have demonstrated acceptance of, is yet another launcher, the launchers less elegant to steam, and everyone is just fine with.

It will probably work fine but it's not an officially supported use case of the software. You can't exactly submit a ticket to Valve if something doesn't work right because the game isn't even on their store.