Ben and Uma WOULD NOT WORK as a couple!
I am not just saying this as a Bal and Huma shipper who just wants my ships. I am thinking from a realistic standpoint.
All Uma ever wanted if she got off the Isle was to SAIL with her crew. She wanted FREEDOM to BE an actual pirate and to see the world.
I'm not saying that Uma would be a bad High Queen. On the contrary, she'd probably be a very good one. She was pretty much ruling the Isle stably in D2 in Mal's absence
But even if Ben DIDN'T have Mal and wanted Uma. Uma would. Not. WANT. the stately, restricting sit at a desk job that being High Queen IS.
Maybe at first she would give it a shot. For example, in a situation where Mal didn't break the love spell in D2 straight away. Uma may have stayed with/dated Ben for a while. Thinking it would give her influence and the ability to help the Isle/make sure the Isle is not forgotten again. But in the end, being with Ben would suffocate her. They would last maybe a year at most before Uma broke it off.
She just cannot be the type of girl Ben needs no matter how much she would supposedly care for him. She just can't love him the way she loves Harry or the way Ben would need as a High King.
So I'm sorry Buma shippers, but it's a FACT. Buma as a ship realistically would not LAST even if they did date.