(cw: the second paragraph of this describes domestic abuse in a book)
I'm thinking about this again, and I think one of the biggest places these books fail from a technical standpoint is that they paint themselves in such a corner with the messed up power dynamic that they basically have to deus ex machina the romance.
There's a book I read a year or so ago where the man, among other things, initially approaches the woman under false pretenses for the purpose of manipulating her into a relationship to get access to her brother, drugs her so he can marry her while she is under the influence, drugs her again so he can tattoo his name around her ring finger, and switches out her birth control with a placebo so that he can more effectively trap her in the relationship.
She is understandably miserable for a good chunk of the book, even without knowing about the birth control bit.
And then at some point the book tells us they're in love.
But the book itself hasn't presented a compelling case for love. For lust, maybe. For infatuation. But there's basically no point through his abuse when it seems like they like or love each other, so the book needs to rely entirely on telling us that she loves him and having him save her from a Worse Man so we know that Things Would Be Worse Were He Not Around.
There's a whole subset of romance novels of various subgenres that run into this problem: the man is so awful and their dynamic is so messed up that there is no room in the plot for them to actually fall in love or even like each other. The book essentially runs out of runway and has to just announce to the reader that they really do love each other, promise.
Ignore the fact that we have seen no more than one moment of genuine kindness or empathy from the man. Ignore the fact that the woman cannot physically leave. They love each other. I swear.
I think when you're writing a romance novel, whether it's with this sort of power dynamic or otherwise, one of the key questions you should ask yourself is: why do these characters love each other?
A romance novel is essentially a persuasive essay: you are convincing the reader that these characters are falling/being in love. One of the core plots of a romance novel is, by definition, the characters getting together romantically, which means that for a romance novel plot to be convincing it should convince the reader that these characters would get together romantically, and ideally that they would want to do so.
Do they like each other? Why?
Do they love each other? Why?
How did we get there, and would those steps of getting there realistically lead those two character to get to that point of liking/loving each other?