Several decades ago, I rated airports thusly:
Grade A: Has metrorail (rapid transit) to downtown.
Grade B: Has transit bus service to downtown.
Grade C: Only taxis and limos (add Uber, etc. for the current day and age).
Boston (BOS) gets an A-minus. You have to take a short bus shuttle through mixed traffic to get to the rapid transit (Blue Line) station. That station got relocated a few years ago, not sure whether it is still on airport property. There was a proposal to loop the Blue Line through the airport and then back to its regular route but I really do not favor that, instead suggesting some kind of back and forth monorail or elevated tram. Boston now has a "tunnel bus" that is free from the airport to downtown including connection to other subway lines but that gets mired down in traffic.for the first few miles out of the airport.
Honolulu (HNL) gets a C. Although there is bus service, you may not bring baggage aboard.
San Francisco (SFO) still got my B way back when despite this: I remember the driver of the 7-F (freeway express bus) tell me, "No baggage allowed, take the 7-B" (local surface streets bus). Haven't been back since to see how close BART rail extensions have reached.