Kindle Unlimited is not a good model, though. It is extremely anti-competitive. I'm amazed that it is legal in the EU (no big surprise about the US). You can only enter it as an author if you guarantee that your book is not available anywhere else. If you don't put your book on Kindle Unlimited, however, then Amazon will make discovery way harder and sales will be very low. It's essentially a way to keep books on Amazon only.
On a side note, I once had a book that steadily sold 3-5 copies per month, then ran an ad on Amazon that barely boosted the sales and had negative ROI, and directly afterwards there were 0 sales/month. How do I know Amazon's algorithm hasn't downgraded discovery to make me continue to buy ads? I don't, this is entirely up to Amazon and nobody controls or oversees these kind of things.
On a side note, I once had a book that steadily sold 3-5 copies per month, then ran an ad on Amazon that barely boosted the sales and had negative ROI, and directly afterwards there were 0 sales/month. How do I know Amazon's algorithm hasn't downgraded discovery to make me continue to buy ads? I don't, this is entirely up to Amazon and nobody controls or oversees these kind of things.