It is not a lot easier to get married in the UK -- you (the USC) will need a special visa to do so, and it's both time-consuming and not cheap. As everyone else has said, as long as she makes it clear she is here to get married and then return to the UK for any visa petition, it shouldn't be a big hassle. I work with someone who was living overseas with her fiance, and she wanted to come home to get married then return to her life abroad. Her fiancé flew over with her, and when they entered CBP asked the fiance what was the purpose of his visit. He said to get married and then return to the UK. That was it, no fuss. They had their big wedding and then they went back to the UK. About five years later they moved to the US, all no fuss.
It is absolutely NOT a risk to marry while inside the U on a visit visa. However, it is fraud to enter the US via a visit visa with the intent to stay and adjust status.
SandInToes got a reaction from Crazy Cat in Can we marry here if she goes home after?