We got into an interesting debate in my group this week over a particular scenario:
We were fighting a small group of storm troopers that ambushed us on a boat. My friend jumped off the boat, swam under the boat, and did a stealth check to sneakily come up on the other side of the boat behind two storm troopers without them noticing he exited the water. He succeeded (due to the nature of the chaos of the combat and his skill in stealth), and then requested to slit the throat of a stormtrooper who did not noticed his presence with a vibro knife. The GM requested he do a melee combat roll and gave him a boost die for the surprise attack.
Here's where the issue comes in: from the player perspective, he feels gipped. He requested to do something, and feels there was little to no chance of failure on a successful roll. In his mind, it was very "assassin's creed" style where the chances of failure (or survival) were slim. A success roll of some kind would just result in successfully slitting the neck of the storm trooper and killing him. Rolling two successes with a knife is hardly cinematic in the way the player desired (it did like, 4-5 damage?), and he argued his ideal playstyle was almost neutered (why would I not just play a ranged light/heavy user when melee is underpowered??)
In the GM's eyes, he felt there was an importance in the narrative dice. The boost die was essentially for the jump, and if he managed a crit on the melee roll the minion would be dead anyway and then this result could be from the knife to the jugular (to which the player argued he could do that from medium range with a gun instead for the same effect). He felt it would be too much 'cheese' if all you had to do was be stealthy and you could take out virtually anyone (to which I agree). He said if it wasn't an important character, or if he was an unarmored guard standing watch in a dimly lit area, it would be fantastic, but given the circumstances it seemed too easy to just say "i kill him".
Obviously the issue is between desired cinematics and maintaining mechanics. I totally get that the player feels his playstyle has been neutered, while I also respect the GM's opinion of maintaining some sort of balance.
Anyway my question to you is: if you had a player request this in this scenario, what would you say?