I never have liked Mystery Quilts - I think I do not like giving up control of what goes where. Even when I have a pattern, I rarely follow the direction of what to put where and the final assembly lol. I have been known to purchase mystery grab bags and at least with them, if you don't like the fabric it can be turned into something else (like masks!!). The BOMs that I have done, however, have all shown the whole quilt before starting so I knew what I was getting into beforehand - they weren't a mystery quilt at all.
But now with that being said, I did do a Kathleen Tracy Mystery Small Quilt with Civil war scraps on line at the beginning of all this covid mess and had fun with it! There was only one block that i disliked and remade