Had a friend who's into history point out a load of historical inconsistencies in the show. Sort of ruined it for me because when I've watched in the past it's all I've been able to think about.
Hirst tries to follow history as much as he can, but keep in mind that during that time, much of what is recorded of them, was written by others. He does sway from history in a few areas but overall, its incredible. Rollo and Ragnar were not brothers for example, and years apart. But you would have to read the interviews with the writer to understand, I think.