I think the actuell Problem is that Pyrrha will literally rip Penny into single parts, at most when Emerald will use her ability on her... That might give the crowd (and world) an... really big shocker.

It´s absolutly not relevant if Penny even survives getting ripped into pieces, the Moment the crow sees this, doom will be on the horizon. The entire world would see an beloved Young Girl suddenly ripping and other Person apart, limp by limp. The People will flip out, run in Panic and maybe even trample each other to death.

The Grimm waited for such an tasty Invitation, and Cinder gonna serv it with as much firework as possible, so even the stupidst and inexpierenced Grim will run for it.

Why is exposing Pyrrha's semblance a big deal? Is the Polarity Semblance rare/a big deal? I know it's super strong, but so it Weiss and her runes/summoning, Rwby and her speed, etc.

It's the secret behind her skill, and it's the reason she's considered the top fighter in her age group. If people find out about it, they'll know why she seems untouchable in combat: she's literally untouchable in combat.

Worst case, people will actually accuse Beacon of cheating in the tournament by putting in somebody who can stop any weapon.

It isn't cheating, but a lot of people aren't going to think that a girl who can stop any weapon shouldn't be put into a tournament where 90% of the people competing use metal weapons almost exclusively. Her skill as a warrior doesn't matter if people know she can stop them with a wave of her hand.

She has to make contact with a metal object for her semblance to have power over it. She never actually touched his metal legs, just his shoes.