The games were known to get released quite untested (which, especially in the fourth game, sometimes even prevented finishing the games), but over the years it was fans that released fixes and patches. The GOG version already contains most external fixes, while playing the games in ScummVM solves even further issues. Prior to that, it was highly suggested to collect fixes from places such as Quest for Glory Updates and Patches for Sierra SCI games.
AGD Interactive released a VGA remake of Quest for Glory II as well.