To be clear, Smoulder was never nerfed. Not every hero uses the same damage coefficient when calculating their skill strength. Smoulder is on the low end of this coefficient, because his base damage is high. In other words, he does a lot of damage early on, but doesn't scale as well with gear. This is intentional.

I do agree with the point that having the 6-piece bonus help mages more would be advantageous as it could extend their usefulness. Overall, I think mages are a bit behind at the top levels.