I think trying to 'rank' bust picks as somehow worse than others is a bit dubious.
Any player who ends up simply busting out and not getting minutes doesn't really provide enough sample size to truly say, "This guy is better/worse than this other guy."
And there are a lot of factors that contribute to just how tiny of a look a player gets before being dropped. Late picks coming in to a strong, contending team, are probably less likely to get a significant look-see than late picks coming in to a crappy team. Some really awful NBA teams may give a whole season of rotation minutes to a player who would not stand a chance of making roster on any other team. (We know what kind of teams I am talking about…) Sometimes it is not who the player is, but what team picked him.
Things are also changing somewhat with the increased use of the D-League, with the much better rules for sending players there and back. As well as the increased use of Euro leagues for stashing late pick players.
I think you may see more late pick players get even _less_ of a look at the NBA level since they can now be vetted more at the D-League before the team gives up on them.
I think Boston is ahead of a lot of teams with using the D-League this way and Fab is an example of that. In an earlier era, he probably would have been give more than 36 minutes in the NBA. He probably still would be a bust, but they would have wanted to make sure before giving up on him.
So, while it's easy (and imho, accurate) to say, "Fab was definitely a bust.", I don't know if we can truly say he was more of a bust than say, Royce White, #16 from the same draft who played only 9 minutes, or Jared Cunningham, #24 who played only 139.
LOL - Giddens made it to 247 minutes of NBA playing time in his career. Is he really a bigger bust than #8 pick Joe Alexander who managed to play 745? Or #20 Alexis Ajinca?
And so on. They are all busts. To me, a big difference may just be in how fast the team that drafted them was able to recognize and accept that they were busts and move on.