Primarily tabs losing their position, not maintaining their hierarchy, weird behaviour around moving tabs around, crashing and freezing. Also all-around bad performance. Maybe it's improved since I last used it a year ago, but it was so infuriatingly buggy for such a long time that I'm just not willing to give them another chance. Especially when Sidebery just works (99% of the time) and it's more performant than TST ever was.