When RNG has positive impact? It's always positive for one side and negative for the other, right? I can't figure out positive rng in general, for both sides.
RNG can have a positive impact on games when things are within a player's control, or at least feel as they are within a player's control. As long as the player has a chance to manipulate and/or react to the RNG based events that are going on, RNG can add positive value to the game.
For example: Let's take a look at the Grenadier versus Conscript match-up. Grenadiers have the advantage at long range, conscripts have the advantage at close range. Sure cans *could* beat Grens at max range given a long string of lucky hits but this is unlikely which is why you always see Cons trying to close the gap. Other factors that players can manipulate in this scenario to swing the fight in their favor would be - trying to throw grenades, using cover/houses, or soft retreating to better overall positioning. Basically, in a scenario such as this one the players have a lot of control over the engagement and are able to influence the RNG components in order to favor themselves. This presents a dynamic where the better player is able to out-play his opponent by influencing this RNG in a more favorable manner. Plus, even if the "worse" player gets "lucky" and his Cons happen to beat Grens at max range - one engagement between two individual infantry squads will rarely, if ever, determine or change the outcome of the entire game.
The same principles can be applied to most tank combat (mediums trying to kill heavies being an exception here which is why I'd like to see heavies lose some of their rear armor). Players are able to take advantage of the penetration values over range and/or side armor to try and influence a battle between tanks to be favorable to themselves. Panthers tend to want to stay at max range because of their 50 range and high penetration even over max distance. T34/85s want to close in on the rear armor of a tiger because they'll have a hard time penetrating the frontal armor of a tiger at max range. These sorts of dynamics are good.
Bad RNG includes - planes crashing on your army and instantly killing stuff. There is literally nothing you can do to prevent or realistically predict that this is going to happen. Sometimes a P47 flies over a 222 or 251 Flak many times before finally (or sometimes never) getting shot down. Sometimes mounted tank MGs instantly kill the P47 before the first pass is even done - and where the plane will crash is practically impossible to predict - even if you could it happens so fast that you won't be able to move your squad out of the impact zone. The 120mm is another example of this - even if your squad is moving, with the way mortar mechanics work in CoH 2 there is still a chance that that shell is going to land right on your squad and kill it... so where is the counter-play? Given enough time (not much time at all) that 120mm is just going to erase something with no way for either player to influence that chance or react to prevent that from happening. The smaller mortars are in a good spot because they are more accurate, but less deadly - if you see your half HP squad getting hit by mortar fire and you don't move it, then losing it isn't really unfair - you had a chance to react. Watching your full hp, full man squad die in one second because you're not psychic and able to predict where every 120mm shell is going to land is not fair.
As I mentioned before, vehicle death crits/heavy engine damage/destroy from AT nades and cheap mines are other bad forms of RNG. I've lost games to ISUs getting main gun crit and driving away to fight another day (that's like a 600 fuel swing). Or Katyusha's surviving stuka dive bombs with an engine destroy. Or my brand spanking new, full HP tiger taking a max range, frontal ZiS shot and randomly getting a destroyed engine. Did my opponent REALLY out-play me by that ZiS gun just happening to get ridiculously lucky and destroying the engine of my full HP tiger? No, I don't think he did.
I just want to see Relic promoting the sorts of RNG that players are able to control, the types of RNG that I feel set CoH 2 apart as an RTS. On the flip side, I'd like to see them limit the kinds of RNG that will forever keep lots of competitive players away from the franchise - the types that can swing a game in a massive (and often unfair) way.