As a DM who has had to do this (only once, albeit with a mechanic I wrote myself), my feelings are very mixed.
On the one hand, I very much prefer to never, ever "nerf" player character abilities if possible. I very much believe in under-promising and over-delivering, as it were. Let the player choose something because they see something cool in it, and then give them something awesome they never expected on top of that. It fosters excitement and creativity--because they know that there may be more to what they can do than just the words on the page. Particularly if these are abilities already written into the game, rather than ones I have written myself, since pre-existing mechanics are implicitly "what I am offering to let you play" rather than "this is my best-guess effort at something functional."
On the other, sometimes certain things really do just end up being WAY too strong. In the aforementioned case, I had given the player a cool new stance that allowed him to do more damage. I thought, from the initial stuff, that it would be relatively balanced. It was not. In both battles with the unmodified original mechanic, by the end he was doing like 6x the amount of damage he could normally do, vastly outstripping anything anyone else could contribute. I had intended this effect to be cool, but not "realign all priorities to make our living blender even better" cool, and another player spoke with me privately about feeling overshadowed and unimportant as a result of it. So, although my extremely strong preference is to never nerf things, I had to admit that I had flubbed up here, so I spoke with the player and asked him how he felt about it. We talked for an hour or two, discussing what he liked about the current status of it, and I presented possible directions we could go to fix it. He picked one, and together we worked out a slightly toned-down version. In brief, sans detail, it would be more costly to trigger (meaning he would have to think very carefully about using up that resource for other purposes) and would ramp up more slowly. He felt that was appropriate, and has continued to enjoy it going forward.
TL;DR: I had to do this once in an extreme case, and having to do it at all bothered me. I did as much as I could to work with the player so they were included in the re-design process. In any less extreme case, I instead prefer to build up other PCs and their opposition, rather than pull down someone who has risen too high. I would pretty much never nerf an existing ability, unless it was so ludicrously extreme that everyone could see it was issue but had (somehow) failed to reach my notice at the pre-play stage.
