With the not mentioned massive downside: it only works when you distribute your game through Steam.

I don't think that's accurate anymore. Here's a 4-month-old reddit thread helping someone get an Origin game working with the SC.

You may have to launch the game through Steam on your PC, but not necessarily acquire it through the Steam distribution system.

edit: forgot the link: https://www.reddit.com/r/SteamController/comments/53ftxp/sup...

It's about these new APIs. If they'll be tied to Steam, and games will use them, then controller will be useless when those games will be distributed through GOG and other non Steam stores.