The problem with steel cased ammo is the Laquer that the cases are finished with.
If you shoot alot at one time or do alot of radpid firing, your chamber & Bolt Carrier Group heats up.
This in turn can melt the laquer from the casings & start to gum up the BCG (extractor, firing pin, etc).
The AR style rifles have a floating firing pin. If the firing pin gets stuck in the forward position, it's possible to have an accidental discharge when the the bolt moves the round to the chamber. This discharge can occur before the round is fully locked into battery.
So, yes, it can be bad, if you shoot enough to melt the laquer from the casings.