TST (or one of its plugins) may become buggy and a memory hog if you have too many tabs. Things like dragging tabs stop working if you have too many tabs opened at once
I just wish that, when they integrate a subset of functionality of TST, further extensions can customize its behavior (like currently extensions can customize TST adding new functionality)
TST is one of those things I can't live without anymore. Even if they did implement native vertical tabs, I'm sure it wouldn't be nearly as featureful, not to mention the various plugins for it