I believe that if you think you are not learning anything from tactic trainer, that is because you don't take the time to review why you got each problem wrong and just move on and hope to remember it next time.
At that note, user additions to the analysis and source would be nice because sometimes stuff doesn't come up in the top 3 choices, so I go and analyze it with a chess engine. If I could then paste that into the analysis and source so that other people may have made the same mistake could see why that was wrong too.
one of the limiting factors is that sometimes there are "in-between" moves that are "good", but not "best". but they may still be relevant and are intermediary threats that keep the original tactic in-tact and present, but are not directly related. i'm not sure if that makes sense to anyone but me :) but it's a real issue in generating tactics (which is done in an automated fashion from hundreds of thousands of chess.com games).