i didnt try much Tartarus but the ideia i guess its only to go in Legendary, for "endgame" experience. Tartarus boss was to weak then they buff him patches ago,i dint play since then.
In my opinion, i would avoid Tarturs dungeon for now, that dungeon have some serious problems from mobs getting stuck/being invisible , and crashes.
Edit : Dont forget Tartarus gives random negative affix to the player, from less resistance/ no pets etc, maybe that was the reason ?