Its too soon to say, but yes in general he takes historical liberties with some key details for better story, it stood out the most in Rome series (ofcourse since I know alot more about that period then I do Mongols), the age differences and some what if scenarios he explored that havent been close to verified (to glorify Caesar more) etc. But in general he stays in good territory (mainly annoying thing was age difference he made about Caesar and Brutus, it was unacceptable almost for me to make them the same age, when Caesar was near father figure who was almost two decades older). Rome series in general was fantastic, but I defenitely liked The Mongol series better, I guess it was just ok for you, but for me it was very refreshing reading a historical novel of a fascinating people and conquoring family that I never would imagined reading a good book or few about (historical fiction genre in general is way too western-centric). Plus the best about Conn Iggulden is his vivid battle and grit of warriors, and there's no more suitable topic then the Mongols for a writer who excells at that.
When it comes to the war of the roses, I'm only 50 pages in, so I cant say much, book been snatched by my sister

(4-5 of my siblings read Igguldens work for last decade, so I couldnt keep it around for long lol). Plus I'm not that keyed into the historical details of the period as I would be about ancient Rome, besides knowing the bigger players etc. So I wouldnt think I would be as annoyed as you if he takes too much liberties since I wouldnt notice as much. I generally like reading historical fiction thats pre-medieval times (besides Mongol part of the world hehe). Most else feels too "modern" for me hehe.
But if you are thinking of buying it as a gift to your boyfriend, just do it, for someone who likes Iggulden, no new series of his would dissapoint them at all IMO.