It's important to recognize that A World Lit Only By Fire started out as a biography of Magellan. This would end up being the last third of the book, which is at least reasonably well-researched and is by far the best-written part of it (even if he strays into melodrama a lot regarding Magellan to try and keep things interesting).

However, Manchester decided to expand the book to provide historical context to Magellan - an admirable goal if it had been done well, which it was not. Certain of his assertions are lascivious rumor presented as fact (his coverage of fifteenth century Rome and in particular of the Borgias), others are unfounded opinion presented as historical perspective (his treatment of early Reformation thinkers), and some of his assertions are just openly laughable (peasants going about naked all the time).

Furthermore he spends a lot of time presenting a stereotyped and outdated view of the Medieval age as a whole, including the old chestnut that it was the "Dark Ages" and that everything was filth and superstition and ignorance and disease from the fifth to fifteenth centuries. As far as I know, that's a perception that historians were putting to bed in about the 1920s at the latest. He tars the age with a wide, wide brush, and the result is some very poor history indeed.