During the river battle the Rus fleet is missing in in several shots as if the VFX-team did not add them.
Landing craft where the entire front of the craft drops forming a ramp for soldiers and equipment to disembark and directly storm a beach were not developed until WW2 and are completely out of the question for the Viking era.
King Harold quotes to Ingrid, "The best fruit is always forbidden," referencing to develop a relationship with her. This refers inaccurately to the story of Eve, of which the Viking culture was not fully aware, including King Harold.