Take a look at this thread, courtesy of /u/PapiriusCursor

The gist of it is regardless of whether or not they lost the majority of battles (which is a muddy issue I'm sure), and yes raiders usually targeted monasteries, the vikings were still good enough to bring multiple polities to their knees, carve out major polities for themselves (Normandy, Sicily, Danelaw England, etc), attack far and wide, and be prized mercenaries around Europe (famously incorporated into the Varangian Guards)