I'd add Hoyer and Cassel as successes. There are essentially 3 big tiers of QBs. The "starters" of which there are often less than 32, the "career backups" which are just depth and have no reasonable path to play (think Charlie Whitehurst) and then those in limbo between the two. Those are the backup QBs that you want to have. The ones that are not consistent enough to be starters but show can steer the ship for a bit without taking it totally off course.
I could not care less how people tend to make fun of them, the fact Hoyer and Cassel stuck around for years in limbo between starter and backup shows that they were positive stories. If someone doesn't think that being in the league for that long, having a positive TD:INT ratio while passing for respectively almost 10k and 17k yards is a success in itself then they should check out the average career lengths in this league. Keep in mind one was a UDFA the other a seventh round pick.
That is also the reason I would not add Etling to the list of failures. A seventh round pick QB is mostly someone who gets drafted because there is something intriguing in him but odds are he is a camp body.
Mallett, Brissett and KOC definitely were misses.