These products, like lemishine, have lemon (citric acid) in them. Depending on how much you use and/or how long you leave the brake to soak in the cleaner, it can discolor the metal finish.
That being said, I have never had that happen and the people that have, say it's just on the surface and can be buffed out using a cleaner like windex that neutralizes the acid.
For me personally, I'm not at all worried about a slight discoloration of my muzzle brake. It still functions perfectly.