If you're surprised by multiple enemies you're fucked. It's an ambush and you're most likely going to die.
Doing the above would mean that players essentially get an action for free after the first shot is fired. So were they surprised or not?
Not to mention, if a group of 5 guys (using the same initiative card) ambushes the character and 1st of them misses, the character defaults to failed CUF and drops into the ditch, the ambush ends because the other 4 NPCs lost line of sight?
Naw, that's silly.
I think I'd allow it, but only because failing the CUF roll is more hindrance (take 1 stress, lose your entire next turn) than it's likely to be a help, in the event that you're being attacked in an ambush. You don't get to take cover, you only get to change your stance to Prone, and you don't even get to act to take cover on your next action, which means that unless it's a far ambush the guys are just going to be popping over to finish you off.
Also, in order to qualify for the CUF roll you have to be hit, (even if you don't take damage from it) which means in your example you'd just keep strolling through the Kill Zone.