I say this only because we've seen GoG get plenty of horrible "classics" but that's okay because that's helping the GoG/DRM-free community out. Maybe I'll prefer GoG someday, who knows, the way GoG is currently run is unappealing to me at best. If major changes are made whilst keeping the Drm-free cause then I may consider switching.
So what horrible classics have you seen on GOG? Except Master of Orion 3 maybe, but that's arguably neither a classic nor truly horrible in the way a Ruins of Myth Drannor is. *shrug*
Yeah, there's a massive difference between MoO3 'bad' and PoR2 bad (though to be fair, the most infamous PoR2 bug was actually a bug in the 3rd party installer software rather than the game itself). I actually liked MoO3 well enough, personally, as I hadn't played the first two games. OTOH I disliked Jade Empire strongly, though it's technically absolutely fine and a lot of people like it and found Rebel Assault 2 to be boring and have rubbish gameplay, but again some like it and it wasn't technically poor or anything.
GOG does sell a few games which are broken in certain circumstances- eg The Saboteur won't work if you have more than 4 cores- but they are labelled as such. I can't think of a single genuinely bad old game that is on there, and several newer but unsupported games have even been culled from their catalogue recently. About the only thing that seems to get you culled from Steam's catalogue is being mean to Gabe. There's certainly no way those 5000 odd 2017 games are all going to be supported and stable now, let alone in a year or two's time.
If anything GOG can be criticised for keeping things too up to date- which has led to some games losing support for legacy OS like XP.