The biggest issue by far is how slow it is. I don't mean bubble sort slow, I mean deliberately engineered quantum bogosort. A cache-less refresh for me on a blazing fast dev machine takes between 3-10 minutes on a normal day, though it might only take 1m if the internet gods are feeling particularly merciful. That leads to all sorts of avoidance because using it for anything takes long enough to be worth documenting as a ticket in its own right.

Compounding this issue is how many clicks even "simple" tasks take because of their bizarre choices. Basic necessities like changing ticket status can't be done without opening separate pages, etc.

One you've actually managed to create a ticket and assign status, good luck finding it later. Backlogs inevitably evolve into infinite swamps that no one knows the full contents of. Not helping is the fact that the search is terrible. I often vaguely recollect some detail or test procedure that someone helpfully mentioned/documented in a ticket somewhere before closing. I successfully find maybe a third or less of those.

Also it's highly customizable, so any skills or knowledge from one company don't apply to the installation at any other.