So first I'd like to start off this post by making it clear, that this Isn't coming from someone who plays only magicka, and cries about "OP stamina" all day while playing relatively dead/outdated specs and class combinations. I main a stamina nightblade, but have been playing magicka temp as of late.
One of the biggest issues with proc sets that seems to go unmentioned is the fact that you're getting FREE damage without having to build into it or sacrifice much to make it effective. I'll give you an example; Valkyn Skoria, a proc set, but requires the user to create a build centered around using damage over time effects to achieve the full benefit of it. People who run things like viper velidreth widowmaker (in whichever combination) all together, get free damage in the sense that you can virtually proc all of these sets together by doing ONE LIGHT ATTACK, OR ONE BASH ATTACK, this shouldn't happen. An individual should not be able to bash an enemy and drop his health to 30% because of procs, It creates a distasteful PvP environment where everyone chases after the newest most OP proc set combination to use, and it makes landing kills inevitably a game of chance / luck. (even though when you're fighting a one punch build viper-tremorscale-widowmaker stamina dps class theres a 99% chance you'll be melted).
Proposed Ideas I'd have to "fix" or "balance" these sets, is for example, keep vipers proc chance and damage, but ONLY allow it to proc off of direct melee range poison (or critical hits). Keep velidreths proc chance and damage, but ONLY allow it to proc off of dealing disease damage, you get the idea. This way people are required to create builds that synergize with their monster helms and actually requires thought to create, rather than just being able to melt people down because their sets carry about half of their DPS.