@matic22 said:
It's stuffing your ears while singing. Multiple singers who don't use IEMs spoke about this. Sammy Hagar is one of them. They simply cannot have something in their ears. A friend of mine bought a wireless IEM system when we used to play covers and he went to the stage without trying it. Mind you, nowadays you have phone apps and you can adjust your IEM mix yourself. But he simply couldn't sing with them.
Also, part of Bruce's singing style is opening his mouth really wide and singing with power. Massive jaw movements can compromise the sound with IEMs. Stage and audience perception is another thing that is really different with IEMs, even with ambience microphones. I love IEMs and I use them most of the time when I play and even when I tech for a bigger band. But I can see why someone can have a problem with them, especially the singer.