Theoretically if you have a, say, knife and are standing behind a door and someone walks in and you snap shot them at 4tus + say 3 tu penalty (7 total) and the guy walking in tries to use a full auto machinegun fire (25 tu)... should he fire first, or get stabbed first? (Not even sure if the knife thing is possible, just making an example). Would the knife get multiple stabs before the machinegun started firing?
It would probably go like this:
1. (If you had sight of the MGunner before he entered the room for 7 TUs) -> ONE knife-stab
2. (Since it his his ACTIVE TURN, he can do anything he wants!) -> full-auto MG on knife-wielder
3. (If knife-wielder survives lol) -> a ton of knife-stabs at MGunner
If it were the other way around (Knife-wielder walks in) it would be like this:
1. A ton of knife-stabs at MGunner
2. (If MGunner survives and knife-wearer had used ~25TUs in sight of your team) -> full auto-fire at knife-wielder
Of course, turtling is still perfectly possible if you just pop out of a corner and nail the target, and even safer than reaction fire, so I'm not sure why reaction fire is punished so much.
In most cases that will work pretty well. Sometimes it really doesn't since aliens are suicidal bastards. E.g. what good is hiding behind a corner with no working RF if an Ortnok just closes in on you and finishes you off with a brutal head-butt. ;-)
I have killed an alien recently with it, and man was I surprised when it happened, so it does trigger eventually, but nothing I would rely on.
Lol. I actually saw my MG-dude fire off a 14TU burst in RF once, killing the alien who was like 40 yards away halfway under cover behind a piece of Ufo-junk.
I also was very surprised when my crappiest soldier with an assault-skill of 21 fired off an aussaultrifle RF-snap shot through two doors of a house and killed the alien that was walking behind the house!
Then again like you say, you just cannot rely on it. The same two soldiers have also been kneeling with plain sight on alien "walk-bys", that MUST HAVE used up at least some 20TUs, and they did absolutely nothing.