I thought this was a trick question, because the first response is: there is an upside? Really?

I hate them. Just book on the airline you are going to be flying on. If it is too complex, use a TA. If the TA finds it too complex, get a new TA.

BTW, the reason for my hate is simple: if things go off the rails, it is usually far harder to fix it than if you are booked on the operating carrier and flight number. And similarly, if you want flexibility (like you might get to depart or return early) then they are a PITA.