I've taken the bus between Dover/Portsmouth and Logan more than a half a dozen times with no issues. I think once pre-covid there was overflow at Logan, but the driver said there'd be a backup bus arriving shortly for anyone who couldn't board.
I take that bus from South Station to Seabrook. I've never had a problem. There are about 30 people. When I'm on the bus there are many empty seats.
Thanks! Your answer is awaiting moderation.