I've read a lot of self published books and there's a diverse mix of good and bad. Some are downright lazy, but some are excellent and it's a mystery as to why they've not been picked up by a publisher! I wrote a piece about why writing book reviews is hard, for this very reason - because I try to support fellow authors, then sometimes get a shock when I dip into their book! https://writingcooperative.com/writing-book-reviews-is-hard-3cfa878b0b79