Standard deviation, mentlegen. I'm pretty sure the RNG follows this as well...
So every blue moon your lucky mortar gets that wondershot which wipes the whole squad. And just as often there should be a critical miss when it could have killed the whole squad but it just didn't.
Game usually has two players, or at least two teams. A RNG event is a loss AND a win as long as you spectate without favourite player. It's not that YOU always miss with snipers and the ENEMY always squadwipes your teams. It goes other way around too.
When you think of it, every mortar shell has RNG already calculated in. Most of the shells just fall in to the "normal" area and behave as they're supposed to do. Same goes with everything else.
I liked Wardonnos post, but:
Basically, the general idea is the RNG is fine as long as it does not cause to much of an imbalance between players. The main problem is that COH2 stresses the importance of unit preservation, however, making it that when RNG kill valuable units frustrating.
The RNG doesn't kill units, the ability does.
Those "unfair stuka loiters" are not a RNG killers, but a game mechanic. If you see stuka, you should move your infantry, or risk a wipe. Same goes for all barrages and offmap abilities. Don't blame RNG, if you stick around you're bound to get hurt. Bad.
Game doesn't simulate ricochets actually hitting anything, dud ammo, human error (such as totally screwing up artillery barrages), misfires, engine/mechanical failures, bogging down, detracking etc... All this would bring even more RNG into the game and spice things up. Maybe they're left out for not to totally outrage the sore losers and chess/pokerplayers.
TL;DR
RNG is everywhere. Abilities and gameplay wipe units, not RNG.