The thing about ship balance is that it has to held against balancing of crew sizes.
Which is to say that in order to say sloops and galleons are equal, something about galleon has to be bad enough that being 4v1 isn't actually an advantage. That premise extends to all matchups. A crew of 1, 2, 3, and 4 need to all be made equal via ship design.
What that means more directly, is that being able to do more things at once needs to somehow not be an advantage. A solo slooper can only do one thing at a time. A galleon crew can do all the things at the same time (repair, maneuver, board, and man cannon).
Let me take a second here to plug that I've always wanted Rare to add another ship actually designed for solo, since sloop can't simultaneously be balanced for both 1 and 2 crew members.
A lot of people say ships are very well balanced. I honestly don't see it. I've been playing on/off since beta and don't remember ever feeling like ship designs managed to cancel out the variance in crew size. I still see galleons as best and sloops as worst not because of any ship detail but because 4 beats 2, and as long as the galleon crew doesn't suck I expect them to win every fight no matter how good the opponent is. To be clear, I have sunk galleons as a solo sloop, but my response to doing that has never been "I pulled off the thing", it has been "wow they failed really hard at the thing"
It's a side note but I will say it feels weird to me that only galleon allows having the whole crew on cannons at once. I'm not sure I follow why cannon counts are 1, 2, and 4 instead of 2, 3, &4 or 1, 2, &3