In my games, only if i send them at worthles units do they come out without a scratch. Try to throw a valor 0 beserker unit against some huscarls who have atleast 1 valor. Your berserkers may end up killing a lot, they may even route them, but they will have taken heavy losses themselves.

When i get my berserkers ready for combat, and if i use them like they were used back during those times, not many survive their third battle. A unit to me is only good for three battles before it has been torn down by losses. plus any sort of cavalry will rip through berserkers without much trouble.