Reason - mono montana, Robb CR, Artus, , Mono Skeelz, Mono Jungo, Mono Roots, Mono Piranas Dreen, Miho, Quibik, Lena, Desomond, Mona, Tula, Benicio, Jean, T Gaank, Haakan, and ALOT more counter him.
When he was released, he was extremely overpowered. Then UR released a ton of overpowered cards (cards that took the place of older cards) and now he is fair. Please note i didn't include soft counters like monosential, or cards like stella and corrina, who can stand toe to toe for a couple of pills.
Without him, Berserk are currently one of the worst clans, next to La Junta, whom lack solid DR and Protection: Damage, but that is another thread entirely.
I made my ELO record (mid 1300s) with mono Berzerk during the time when Spyke was allowed and to be honest, I felt kinda bad when I won a game by turning the match around with Spyke. The fact that UR would combine -4 opp power min 4 with stats like 7/5 baffles me.
Some people have asked about Lou. I believe the reason she was banned was her synergy with the many good 4 damage cards in Roots in the days when ELO still had 12 life instead of 14.
I suspect the reason Lou is still banned is because of all the +life cards that have been showing up in roots lately which still makes her dmg danherous even though the ELO life is now set to 14. This coupled with the fact that decks like a Roots-Jungo Hybrid would switch to Lou when Askai is banned probably is what's keeping her perma-banned in my opinion.