Take everything I say with a grain of salt. I may be wrong.
Those debuffs that fail have a 15% chance of failure, disregarding potency and tenacity. Move's like R2's basic have the chance to try to apply the stun, and then the chance to stun. So an 80% chance to try to stun, and then it's got to get past the 15% chance of failing to stun.
DN and asajj don't apply debuffs, they just wipe buffs off. Because it's not a debuff, there is no chance of failure.
Someone else should probably confirm what I said, but that's how I think the game works.
