Stephen E. Ambrose comes to mind. He's not specifically panned for being a bad author quality-of-writing-wise, but in recent years (his final years and after his death, roughly 2000 to the present) serious accusations of plagiarism and a lack of objectivity (Ambrose wrote "popular history", general material meant to be consumed by a wide audience, more so for the narrative and less so for serious academic analysis and facts) in several of his books have come to light, and with that his posthumous reputation has taken a severe hit. u/Georgy_K_Zhukov essentially summarizes his issues here.

Not military historians but Doris Kearns Goodwin and David McCullough come to mind. Goodwin was found to have plagiarized parts of an early book on the Kennedys. McCullough is dismissed as a hack by scholars. In their defense, though, they are gifted writers and speakers so they have made many people interested in (great man US) history who otherwise wouldn't have been. I personally have mixed feelings.