I like tree-style tabs, and I use it when working on projects, but I find that it is a long, long way from what it could be, which is probably a direct result of the lack of competition

the settings UI is absolutely awful;

depending on which appearance you choose, it either looks ugly or blurs together;

it frequently has visual glitches;

could certainly be more customisable appearance and shortcut-wise;

doesn’t allow you to have a title or marker as the top level tab;

as far as I can tell doesn’t let you save a session of tabs;

has an unpleasant glitch where if you drop a tab in the wrong place it’ll disappear behind the current window and never be reachable;

the new tab behaviour irritates me;

and probably a few more complaints that I’d have to actually open it up to remember