I loved the Flashman novels. They were extremely accurate. Royal Flash was one of my least favorites, as it was so heavily derived from Zenda. The film version Royal Flash was pretty good, IMO. I though Malcolm McDowell made a splendid Flashy.
A lot of people agreed that Royal Flash was the weakest of the series. The movie had a stupendous cast, and Fraser had done a very successful screenplay of The Three Musketeers, though The Four Musketeers was less successful.
For most of the audience, there was just no familiarity with either the characters or the history. Making matters worse, the timing was of an action comedy, which is a half-tone off from a historical character comedy.
Their best bet would have been to start from the start with Flashman. Even though it would have stretched into two movies, or even a miniseries, if it were done today, the background of historical Afghanistan would resonate. It would need a masterful screenwriter, however.