For your ebooks to be available for the Kindle Unlimited program, they must be enrolled in KDP Select. That means, that if you want to be part of Kindle Unlimited, you must give Amazon exclusivity. That may well be a deal-breaker for many authors. Some authors make more money on their titles on the other ebook retailers in aggregate than they do on Amazon.
In Kindle Unlimited, you get paid when someone reads over ten per cent of a downloaded ebook, out of a pool of funds. The big question of course, is how much will you be paid per book?