They weren't particularly. 'Vikings', if anything, sought to avoid actual combat as much as possible, to strike at undefended targets or isolated garrisons. According to Asser, the great strength of the Danes was in their ability to manoeuvre rapidly, and their use of widely dispersed raiding forces to force the English to split up their forces and tie down vast amounts of manpower, so that the Danes could then bring larger and less exhausted forces to bear in actual pitched battles.

I wrote an answer here that went into a lot more detail about the Danes in England from the 870s to the 920s and examined their track record in engagements against the English.