I don't think the Vikings are historically overpowered. The Anglo-Saxon Chronicle refers to them as "The Force." Any acount usually begins, "And they met The Force at..." and ends, "And the King was driven from the field with (insert horrifice casualties here."

So the Saxons saw the Vikings as unstoppable supermen, until Alfred and he almost lost. The Huscarles were the elite, the problem is that in VI you usually only meet the king and his son, so all you see are the Huscarles. As to the Saxons and Angles having them, well the Vikings get them straight away. Over all its probably the most accurate TW set up.

Gameplay is a different matter.

Oh, and the Saxons weren't the majoriety in England until quite late, in 600 AD 35 men was considered an army. The Majoriety of the population were Romano-British in pretty much the same state as the Saxons were when the Normans took over.