Very error prone.
For example, if you have a window with multiple groups and close it by mistake, when you restore the window, the tabs are reopened but are no longer in their respective groups...
Also, it keeps polluting my Downloads folder with STG backup folders...

Please add an option to change auto backup folder. It's a bit annoying to see it in the Downloads.

Doesn't work - I add tabs to a group and they disappear when the tabs are closed. Also does not add pinned tabs.

Questionable decisions:
1. removing groups will close tabs (!?!)
2. once I opened a group, I could not find a way to return to the non-grouped tabs.
Also, you cannot have multiple groups opened in the same browser window.