* if you use this, you can’t use any other sidebar at the same time

* you need to put in custom userchrome.css to hide top tabs. Once you’ve done this you can’t easily swap between vertical and horizontal tabs.

* it’s ugly and, while not exactly buggy, there is a lot of UI weirdness that comes with it being a workaround

I still use it and have done for years but native support would be a big improvement.