We actively use ours and it's never been an issue. Simply, lift the tray on the chair behind you, slip the harness over, and close the tray. Normally, families with small children get to pre-board, so we have it setup before the other passengers have arrived. Even after, we've never had any push back.

Notably, though, German carriers like Lufthansa do not allow it, which I believe is due to German regulations. Outside of that, we've never had an issue in either the U.S. or Europe, but occasionally we need to show the airline a copy of their policy from their website on one of our phones. They weren't mean about it, but may not have seen one. Generally, the senior flight attendant is experienced enough to know the rules.

Our kids were huge, so we used it from before they were a year old and it has been very helpful.

It did work with the tray tables at the time (underneath, while open or shut). This was about 4 years ago when we last used it. Passenger behind us was more curious than upset.