I use them, or rather, I have used them. Don't think I am currently, mostly because there was only two available when I was moving from 2.5 to 3. For me, what would be really useful would be to have a track be able to part of more than one group, especially for scripting. For instance, I might have a Group for drums, but subsection Groups would be snares, kicks, hihats, etc.
But I've basically done that using scripting. I have a button for FocusLockDrums, FocusLockKicks etc. From there I can fade, shuffle, whatever.
The great thing about FocusLock is it can be dynamic, right? You can decide, I want to fade Tracks 3, 7, and 8, FL them real quick hit fade. Though it isn't quite as quick now as 2.5 because you have to select the track, then hit the track name whereas before it was just hitting the track name. Suspect that is a Juce thing.
Groups with their own Track Sync Master is an interesting idea, can see creative potential there, complications too haha. Will think about Groups some more, give you more feedback.