Never had a problem with this myself but the bike shop I used to work in were adamant that muc-off was kept away from disk rotors/pads and rims with rim brakes. They might have just been overly cautious, as I think they only used isopropyl for discs
I can imagine that in a shop where you have the right tools for things (isopropyl) it makes sense, but same as Howard, I've never had any issues using MucOff on my disc braked bikes
Muc off is fine, yeah.
The stuff it dislodges from other bits on your bike might not be fine. Maybe that's what happened to Turkish.