The editor is Windows only, but there will be Mac and Linux versions of the game. The Steam controller is supposedly effective even with mouse and keyboard heavy RTS games, so an RPG shouldn't be a problem (I'm assuming Steam will be releasing a separate controller).
As for native support, Larian did post a topic some time ago to gauge how much demand there was for controller support (
Good isometric RPGs with controller support?). They didn't announce any conclusions; presumably if they did decide to add controller support it would be after release.
