Bos has about 17 flights a day to EWR not all UAL but most are. United serves them all with 737/320 and are full to the point it's hard to commute on most of the time. Usually try for JB because there is always a ton of room on those flights. UAL doesn't serve BOS with FFD airlines because they can do it with mainline.
I have been on multiple JB flights with at least 8 UAL pilots plus FA, because htey couldn't get on UAL flights.