First impressions: I'm just now testing Tree Tabs, as a long-time Tree Style Tab user who's followed Tree Tabs from the first Vivaldi forum posts. It has some very nice features out of the box. I love the groups, unloading, and session backups, as well as the auto-correcting tab order for keyboard shortcuts.
Additional thoughts: So far, pretty awesome. Literally replaces three extensions, and very nearly three more, at least. Groups, tree-hierarchical tabs, and tab search are fully replaced, and it almost replaces a session manager, tab-unload manager, and an undo-close menu.
If wishes were... : It'd be helpful to have the session feature save automatically every x minutes and allow selection of a session to load. Tab-unloading would be more useful if it could be automated on a timed basis with a whitelist. Undo-close would be nifty if it had a list of the last few closed tabs, but it works fine as-is.

Bugs: None found yet?
Noteworthy: The audio icon only appears on that specific tab, even if it is within a collapsed tree. This can be a bother if more than one tree or branch of possibly-guilty tabs exists. It might be better to display the icon on the nearest visible parent tab, if the playing tab is hidden, as well as on the group title, to aid in locating an offending tab, especially in cases of sneaky, ill-behaved ads, but also for those who (over)(ab)use YouTube and the like.
Edit: Found the sidebar unresponsive at times. The problem was predictably resolved when I disabled my session manager and a tab-unloading addon. Seems that many (most?) things that interact with tabs directly may interfere with Tree Tabs.