Mongostat indicates many things, but when it comes to a replica being behind and in trouble, it's lacking details in a bad way.

When a replica is being re-sync'd to primary (after a startup, for instance), is shows in state 'UNK'. This is different from the state UNK that exists when the shard is too far behind and will never catch up.

This indicator is not helpful, I have to go to the box and tail the log, and even then it's not for-sure I know whether this thing is catching up, dead forever (too far behind), or what.

This case covers enhancing mongostat with better indications for what state a replica is in and, optimally, what progress it's making in a resync process. (% done resync'ing, ETA when will be done resyncing, etc.).