GOG isn't exactly DRM-free these days. The platform doesn't add its own DRM, but it does distribute games with DRM elements. Their stance seems to be that, so long as the game is playable offline in single-player mode, it is acceptable.
And yet Steam is incredibly popular despite the DRM because it's unobtrusive. You don't get punished for replacing your computer every couple years and needing to reinstall your games.
I'm not sure I've ever heard of someone having a technical problem with Steam's DRM, I've only heard moral/ethical/worried-about-what-happens-when-Steam-disappears problems.