rob: It’s a B’feirush Rama in regards to Karaites. They are all assumed to be Jewish. Therefore, they can accomplish Kiddushin. However, their Gittin aren’t Kosher. So any “divorce” leads to Mamzerus.
R’ Ovadia discusses the reliability of the Radbaz with the 500-year-gap and the reasons for and against. See his T’shuvah.