I've been playing PSIV - End of the Millenium, and I thought that "chance to instantly kill" moves were fairly worthless, up until I got to Nurvus.
At this point in the game, you have to regularly fight high-HP mechanical/robot enemies. Some of them, like... uh... Bulzaren? Or something? The one that has a ridiculous amount of defense and floats with rockets. Anyway, some of them are difficult to defeat with conventional means. So I began experimenting with the instant-kill moves some more.
One thing I began to notice is that Demi's Spark skill seems to consistently kill these robotic enemies. Out of about 15 uses against robots, only one of them failed.
Then it hit me: Duh, she's an android, so she uses an EMP (or maybe hacking) to disable other robots instantly, that's why Spark works so consistently. In game terms, they probably doubled the chance for the skill to work against robotic enemies.
To contrast, Gryz's Crash is very hit-and-miss across all the enemies I've tried it on, and Rika's Eliminate has literally never succeeded, ever.
So my question is this: Does it matter what kind of enemy I use the different instant-kill moves on, and do different moves work better against certain enemies?
EDIT: I should add that before I went to Nurvus, Spark seemed about as helpful as the other instant-kill moves (that is, not very)