Well from what I have noticed more than 50 % have been horrible calls. At the speed these guys are playing at the refs seem to flag any hard hit. One in the Arizona game was a clear shoulder strike to the receivers chest, flagged. Plenty more every game. But reviewing them will slow the game even more imo.
Why not stick with the lowering the helmet and using the crown, right now if your head goes down at all (which it has to, it's connected to your neck), the flag comes out.
Funny watching the players, many looking around trying to figure out what they did wrong. Not against the intent to protect, just need to figure out if they are flagging legal tackles.