You can add any property to any gem.
How abusive could that be?
You could put cast on hit/dodge on panic attack though eventually the cast/attack limit would put a stop to that.
You could put cast on death on spells like Requiem and Divinity. I am assuming these spells are not intended to be able to be cast on death
Basically invincible team via spell gems if this is not fixed.
Fixed for the next patch, thanks!