@Ben - Is that $54 Brightline ticket in the example you mentioned for their Basic fare or their regular Smart ticket? If the $271 JetBlue ticket includes the Brightline Smart ticket, then that would come with seat selection as well, which the Basic fare doesn't allow. Smart fares are typically $20 more than Basic fares. In this case, the JetBlue ticket is only $40 more, which is still more expensive. But having looked at pricing between Orlando and Aventura on Brightline, tickets are typically more expensive than $54 unless you book over a month out. And if you book through JetBlue, the biggest value I see is that the train connection is a part of one ticket, so if your flight is delayed, you don't have to book a new train.
Brightline previously had a program where if you miss your train due to a (separately booked) flight, you can show proof and they will waiver the rebooking fee and accommodate you on the next available train. Not sure if this is still available. Previously, all Brightline tickets could be canceled for a credit up to 10 minutes before departure, so I would just cancel the ticket if I was running late for any reason and rebook on the next train for a fare difference. I often booked last minute Brightline trains so the price for me to rebook wouldn't be anything extra (since tickets go up in price 2 days before departure).