Yes, there's randomness. But it's not a 30% random effect no matter what, you can game it. If the enemy has 60 health left and your ability with the CC does 54-64 there's a 50% chance that you won't break his armour, so you might want to do something else to break it first. You can take the risk, but it's not a 50/50 no matter what. Critical hits are the same thing, let's say you have a 30% chance to get a critical hit, and you know you only break the armour with a crit? That means you know you're taking a big risk trying it. Dodge? Again, same thing, you can see your odds of hitting the enemy and can manage risks accordingly.
In D:OS1? It's always random, can't play around it happening, can't expect it to happen for or against you. It's always random even the first turn of combat. In D:OS2 you have a lot of control over what's going on and risks are those you take, not inherent of the abilities themselves.
