ewiles: I'm really surprised with the number of blatant quest bugs I've seen in this game. Some of them are totally unavoidable and happen no matter what you do. It seems the developers should have caught these with a simple play through. I really hope these "failed" quests somehow get retro-actively fixed and marked correctly through the next patch.
There are some annoying ones but overall I only experienced a small handful and none of them completely broke my game. I assume you've never played Skyrim? :) My Skyrim playthrough had not only quest bugs regularly but totally game breaking "start the game over again" type bugs which would have completely ruined the game entirely if it wasn't for Google and special game console commands you can throw into the console to reset quests and fix glitches.
Now that I've completed both games, if I were to rate the two games based on quest related glitches with 100% being "most stable" and 0% being "least stable", I'd rate The Witcher 3 at 98% and I'd rate Skyrim at like 70%. Outside of quest breakers/glitches, if I were to rate them on overall stability versus bugginess, I'd rate The Witcher 3 at about 95% stable and polished and Skyrim at about 40% stable and polished.
The Witcher 3 is actually one of the most stable games of this size and scope that I've ever played despite the various bugs/glitches I encountered, and they're only likely to fix many of them over time and make it even better. I'm quite impressed at how stable it actually is.
Having said that, I too do hope that the quest related glitches get fixed in the next patch, in particular the "Sven wont stay dead!" bug, and the "Shortcut" quest bug, as well as the druid that has a "!" over his head and wont ever talk to you bug. :)