Instead of looking at the Vikings as infantry, it might be useful to see them as a sea horde as opposed to a steppe horde.

This would give the Vikings the similar features to the steppe hordes of extreme mobility compared to the sedentary populations on whom they preyed.

This mobility gave both the steppe hordes and the Vikings the ability to choose their points of surprise attack, to retreat rapidly if locally outnumbered, and to strike somewhere else rapidly. They could concentrate dispersed forces quickly to mount more powerful attacks, and disappear suddenly with their plunder without fear of pursuit.

As a sea horde, motivated by plunder (and also trade), the Vikings would not stand alone. The Barbary states, pirates from Ancient Times to the Great Age of Pirates, early Dutch and Portuguese seafarers in the Indian Ocean might be seen as somewhat similar.