I'm just an amateur on this one, but I was given to understand that "professional" might not be the best term for them. They were often referred to as The Emperor's Winebags considered their frequent if not constant states of inebriation. Culturally, drinking often was not out of the norm.

But we're talking about primarily Swedish and Danish mercenaries. They could go serve for a couple years and go home as very wealthy men, so you got plenty of experienced soldiers willing to sign up and throw themselves into a fight (to be distinguished). Rather than being professional, precise, or even particularly well-regulated, they were valued both for their ferocity and the psychological effect this had on the enemy. They were slandered as axe-wielding barbarians often, and they came to take pride in such slurs, seeing themselves as a breed apart from the "civilized" people they found themselves amongst. The reputation as barbarians was capitalized on, with outlandish armor, great feats of strength, and unswerving loyalty to the man who was paying them to fight and kill on his behalf.

If they were followers of the Cult of Odin, which is probably likely for career soldiers, death was supposedly not to be feared since if you died fighting the enemy you got to go to valhalla. Consequently, they were recorded joining battle with reckless abandon by the Byzantine standards.

If I had to generalize, I'd say they were effective because they were considered fearless by the standards of both their allies and enemies.

Hey Shlin28, I'd like to hear your opinion on the matter given that this is your area of expertise. What do you think made the guard so elite?