A galleon requires 3 competent crew members to operate effeciently. A Sloop requires 1. It is much, much easier to get one competent crewman than it is 3, and easier still if you are that competent crewman.
You have to consider that a galleon takes more people to sail. That's more people giving their time.
Some sloop advantages you missed:
The sloop is shallower on the bow than the galleon. It can sail closer to land without taking damage, and i think there are one or two shoals in the game, though i've not seen any yet. Think I saw them in a video though.
The sloop stands out less on the horizon
The sloop is much easier to bail and detect water buildup.
The sloop is a smaller target, also making it easier to hide.
The sloop can decelerate for manouvres much quicker.
The sloop is faster against the wind than the galleon, giving it a guaranteed escape route at all times.
But the main point to consider is the first. Finding 3-4 competent crew and getting them all together in a gaming session is difficult. Each player not helping the ship function can pretty much be ignored by a good sloop. wide eye of reach shots and poor ramming attempts aren't really a threat. People waiting for a shot to not line up are no threat. And if you are a 1-2 man crew against a 3-4 man crew of competent players, you can't really complain about disadvantage, since 4 people are always going to have the advantage of numbers over 2.