I was about to type my thoughts into why players think BK > GM as a flank to Telluria, but I was beat to it:
Though I don’t really know which one is better, I do agree with the above reasoning.
I usually bring 2 cleansers against Telluria+Vela because water damage on fire is brutal, and I really need to cleanse the mana debuff or I’m toast. I can’t rely on just 1 cleanser to have that job all to themselves.
But against BK + Tellu + Vela, it gives me pause and I have to think more carefully about whether bringing a dispeller is worth giving up one of my other heroes.
There actually already exists a cap on his attack buff. The berserk attack buff is the same one Brienne, Namahage, and Khiona use in their skills, and all of theirs are set to increase by a maximum of 5 times (capped at 145% attack up).
Personally, I think 3 turns is enough on the 5*s who have this buff. On offense you can already use someone like Wilbur (same color and speed as BK) or Aegir with them to max it within the first few turns: Damage Share ensures that each hit will increase it on every ally who has the buff. On defense, even your off-color tiles increase the buff so having it last longer would heavily punish a bad cascade even more than it already does.
Just my 2 cents but I think 3 turns is fine.