Lack of keyboard shortcuts to navigate and manipulate tab groups. Lack of tools in general to manipulate tab groups. Where are my options to open a bookmark folder to a tab group? Save all the tabs in one group as bookmarks? Save tab groups for restoration later (without making bookmarks)? Move tab groups between browsers? Etc, etc.
Now that's just getting picky, keyboard shortcuts I could care less about, the bookmark stuff done manually isn't that much a time waster, most of these complaints are just sheer laziness than anything really. Especially when moving stuff around is as simple as clicking and dragging, and bookmarking is a keyboard shortcut (there you go!).
The only one that I actually agree with you with is the save a group for later, which would be nice. But, if I want to save a group for later as of now, it really isn't all that hard to just keep the window open

Not to mention that FF already gets clunky with more than a dozen or 2 dozen tabs open, so the utility of tab groups is pretty limited until they fix that.
I've had close to 30 or more tabs open at once, and I haven't seen a performance decrease.
Note: This is also while I had steam and the new Total War game running in the background.