I was reading that the reason the Tzitz Eliezer considered the status of Ethiopian Jews to be a safek was the fact that the Radbaz mentioned in one of his teshuvos on the subject of Ethiopian Jews that it was common for them to take captives during wartime. And given that the Ethiopians were ignorant of torah shebaalpeh, a geirus performed under these circumstances could not have been legitimate.
I was wondering if any poskim extend this logic to Karaites considering that there must have been times at various points where the ancestors of modern Karaites converted people for the purpose of marriage. But most of the material I've found on this subject deals with whether or not they have a safek of mamzeirus and assumes implicitly that they are halachically Jewish. I'm very curious about this.