The cast is packed with talent. The cinematography is excellent. The stakes couldn't be higher. In these regards, it's a success as a drama.
The issue is everything else. The script couldn't be more boiler plate. It even has the trope of ending some sentences with "was it not" just to make it sound old timey. This is a cliche that has been used in numerous times in poor historical dramas. The actors are good, but they still have to spit out the lines that they're given and the script isn't as strong as the performances.
Then there is the history. Let's just leave it that despite this being a genuinely exciting time with larger than life characters and a genuine series of epic and exciting events. I have no idea why the writers just decided to make most of it up.
We have Anglo-Saxons wearing tartan, the armour is largely wrong as our most of the architecture. Battle tactics and interactions between key people are nonsense. The question has to be who is this for? If you're a fan of history, get it right. If you can't be bothered to tell the historical story then write a fantasy like the Witcher. This is an example of a project that will keep nobody happy.
I have literally written a book about the complexities of bringing the past to life. If you're interested, it's called- Hollywood and History by Jem Duducu. This is one of these examples where it has a veneer of quality to it but the reality is this is as high fantasy as the movie 300. It's a solid enough drama just don't call it history.