Given that they were supposed to go into battle wearing just skins, battle in frenzied manner (potentially drug induced) and be immune to fire and steel.
Were these troops real and how often would they be deployed if they were?
Assuming they were real, what about them was accurate, and how would they enter the battle trance?
I have seen many different theories, mostly speculation, as there does not appear to be much if any, concrete evidence. Here is an archived thread on berserkers from this sub.
https://www.reddit.com/r/AskHistorians/comments/2ega37/how_did_vikings_go_berserk/
EDIT: answers provided by /u/Aerandir and /u/Searocksandtrees