Nerfs negate players' investments into their characters.
If one skill out of ten is superior to the other nine, the solution is to make the other nine good instead of making the superior skill suck as much as the other nine.
If making all ten skills good puts too much power imbalance for the player, the solution is to counterbalance by making the enemies tougher, as all skills are only as effective as the threats are susceptible to them.
Moreover, nerfing abilities in your mod will make new players wonder why they should bother using it if their character suddenly sucks with the mod rather than playing without it. "Why doesn't VH work as good here? What the fuck is this? Fuck this mod.", is what they'll say immediately before deleting it.
Never nerf anything. Ever. Ever. Ever.
Late edit: I apologize if this post sounded childish. I just hate nerfs of any kind in this kind of game, because I just see the tendency snowball. "If X got nerfed why not Y?" is how that logic comes, and ten adjustments later instead of having 1 good skill and 9 shitty skills we have 10 shitty skills. I'd prefer to have 10 good skills.