[Kuiper]

Your intuitions are correct; romance novels and thriller novels tend to be the most successful genres on Kindle Unlimited. Different pricing/distribution models attract different kinds of customers. (On the other end of things, I have seen engineers pay $30+ for PDFs about technical subjects without hesitation; I don't often see people spending that kind of money for romance ebooks.)

For similar reasons, the quality and genre of made-for-TV movies currently airing on the Hallmark channel is probably different from what you could experience for the cost of a ticket at your local movie theater: one of these distribution channels caters to people who want to consume several hours of content every day at no marginal cost beyond the monthly subscription that is already part of their budget, while the other caters to people who are willing to pay $15 to spend 2 hours watching a film that a studio spent millions of dollars marketing to them.