It does say clearly on page 206 (second printing)
The only exception would be if a spell says specifically that it is not subject to the effects of a critical hit (but there are currently none).
As usual that's up to the GM, however even with magic armor, it's still doing maximum impaling, slashing, or crushing damage, which in most cases will result in death if head or abdomen or a serious injury for everything else (unless a weedy weapon).