I would say that the quality of the series is very uneven once they reach the New World, and some of the books aren't very good. But the issue isn't the plot being like a bad romance novel. How is the plotting in, say, The Fiery Cross or An Echo in the Bone similar to what you'd find in a (poorly written) romance?…

There is some logic, but the characters themselves aren't 100% certain what's going on and try to work it out themselves. I'm guessing that Gabaldon made up a lot of it as she went along.