I've only read that, generally, Foucault's works are questionable by standards of historians. If this is so, why?
I understand that philosophers read him for other reasons (I think you can consider his work highly analytical even if his historical claims are considered somewhat inaccurate).
I'm wondering why or how historians might read him, if at all
While there is more to be said about his other works, I have written specifically about the reception by historians of Foucault's Madness and Civilization here.
The thing always to remember with Foucault is that he was not a historian, and he was not setting out to write works that conformed to what were considered good historical practices. He aim was to challenge our idea of history.