My personal opinion, based on nothing more than that I've been paying attention, is that it depends on how well known the author is. A new author will get a LOT more exposure under Kindle Unlimited because people can download the book and try out more than the first chapter or so. If the book is good the "people who liked / bought this book also liked / bought...." feature will let the book be found by a LOT more people. And people are a lot more likely to read the sequels to stories they like.

So if you are a relatively new author, Kindle Unlimited is good. An established author like Tanya Allan will make some of their titles available on Kindle Unlimited. Some authors will rotate through making some of their books available for 3 months. This way they attract in new readers, who will then either go buy a lot more books or will hang around waiting for other titles to be available on KU.

On the down side, authors don't earn a lot through Kindle Unlimited. Typically they earn around 0.45 cents per Kindle page read, counted by the last page read. (So if a user borrows a book, skips to the last page, then returns it, the system thinks the reader read every page.)

For instance, the Gaby books are probably sold on the Kindle program that earns 70% royalty less 15 cents/MB delivery fee. This means that Gaby 01 - The Anime days at 262 pages & 4 MB earns $3.83 * 0.70 - 0.60 = $2.081 royalty, i.e. 0.794 cents per page. Gaby 24 - Fame at 345 pages and 5 MB earns $7.18 * 0.70 - 0.75 = $4.276 royalty, i.e. 1.239 cents per page. (Note that most books are under 1 MB in size so they are only charged 15 cents delivery instead of 60 or 75 cents delivery so most authors would earn about 0.17 cents per page more in royalties.)

On the other hand, if they are sold under the 35% royalty and no delivery fee then these two books would earn 0.51 and 0.73 cents per page royalty for a sale but could also optionally be made available under KU, which would earn around 0.45 cents per page royalty per borrow if the book was read to the end.

So a typical 300 page novel by an established author probably sells for around 2 cents per page, which earns around 1.35 cents per page royalty after the 15 cent delivery fee. So the author earns about 3 times as much as they would from a fully read Kindle Unlimited borrow. But if there are 3 times as many readers due to it being available for borrowing then it is worth it.

EXCEPT FOR ONE NASTY PROBLEM. While the book is available for Kindle Unlimited borrowing it cannot be available any where else electronically. (Printed sales are OK.) Which means you can't make the book available for Kindle Unlimited and still have it available for free on BCTS. I think that is why Maddy doesn't make her books available on KU.