I'm picky about the books I want to read. I tried Kindle Unlimited for about a month and I didn't care for what was offered. But, for my dad, he will read anything so it's good for him.
I just have the Libby app, attached to my library card, and I download books from the library. I often have to wait a long time for really popular books but I have so many on hold that something is always coming in.
Get to know Libby. I have found a way to get 3 different library cards so I can reserve (i.e. get on the wait list) for up to 10 books at each. When I find something of interest, like when I read through the recommendations here, I reserve that book. It can be weeks or months before it comes available but that's ok. If you are in the middle of reading something when it comes available you can keep your place in line and pass it on to the next person. You can also just sort through the titles that are available now and find a good read that way too. Not sure what I would do with a Kindle Unlimited subscription as I always have something to read.
The other tip with Libby, recently discovered, is you can borrow and play audiobooks directly through the Libby app. Audio books tend to have better availability too.
One final thing; if you have a simpler Kindle like a paperwhite, you can download from Libby, turn on airplane mode and keep the book indefinitely. That helps if you are really close to finishing a book that is in high demand when it comes due. That will limit you to just the book you have downloaded however. I found this does not work on a Kindle Fire. Kindle books are downloaded to all your devices you have set up on Amazon so you can switch between devices and it keeps track of where you are (wifi has to be on for that though). I had a really hard time switching from paper to Kindle, but now I'd have a hard time switching back I like it so much.