My biggest issue is with JS reservation, if you reserve the JS, there should be an automatic S3 check in. If there’s a seat you can choose if you want the seat or the JS. Sometimes the flight is full and some seats open at the end. I’ve been times seating in the JS while a regional pilot gets the last seat in comfort +.

Also, if the flight is delayed and you want to non-rev but the original departure time has passed, good luck trying to get the agent to list you.