I can imagine not experiencing the global rage mechanic, since heroes being alive for more than 100 seconds is kind of an edge case, and even after that it takes a while before the buff stacking makes it noticable.
The berserkers have their own rage mechanic on top of that, going up to a 4x speed once their hp reaches 25%, at that point they are really running mindlessly into walls a lot. (I initially had them scale up to 10x speed at 10% hp, it was crazy).
I have noticed that the berserkers tend to be a bit of a jump in difficulty. For now I think it's fine, but depending on how things go I might give them a small nerf.