AtlasLoot does not use Ace2; it does, however, use Dewdrop, which requires AceLibrary from Ace2.
For the most part, though, taint doesn't matter. Taint is only a problem if you get "action blocked" messages. Since you're using BugSack, just enable "Filter addon mistakes" and you won't be shown taint messages. If you get "action blocked", then it can be helpful to check taint messages, but otherwise they're not really something you need to worry about.
For most addons, no, there is nothing in Ace2 that would prevent authors from migrating to Ace3. Mostly it's just author laziness ("Ace2 still works so I don't want to spend my time rewriting my addon to not use it"), lack of time ("I don't have much time and since Ace2 still works I'd rather spend the time I do have on more fun/productive things"), or simply the fact that the addon is no longer maintained.
For a few addons, there is the problem of AceOO, which has no non-Ace2 equivalent and makes "upgrading" addons that depend on it a major undertaking.
Which specific addons do you have that use Ace2? I have several, and have never had any problems.