In the ESO Live stream of today they said this is a bug which is gonna be fixed. So after the next update of PTS shields can be critically hit. This fix is part of the imperial city update.
On the PTS it's a nightmare to burn through a diehard shield stacker cause of the damage reduction, especially playing as a magicka build you just can't dps down a shield, and if you manage to get through something it's instantly restacked back to full strength. Just by shield stacking you can almost completely negate crit builds.
I'm glad they're finally bringing this change in.
The Elder Zergs Online
Sainur Ironfist - DK - EU - Ebonheart Pact
Retired
So a critical hit is where you strike a weak point on your opponent. It is a hit they are not prepared for/bracing against/have armor in place to deflect or mitigate.
The criticalness of critical hits is why everyone was using diamonds on their armor. They changed diamonds to cause variability of people's armor layout, but put in the flaw of having shield abilities have the exact same "flaw" impenetrable had.
Bzzt. Ow! Bzzt. Ow! Bzzt. Ow!
Due to a typo in the system, the area was accosted by the Daedric Prince Moar Lag Brawls.
The real issue is and always has been shield stacking. So why on earth will they not simply remove stacking and then adjust the various shields as necessary ?