Basically it comes down to the medium at hand.

Basically, any game has some form of RNG unless its a game where all variables are always known by all parties. Chess is a good example of this; Both people always know where their opponents are, their strengths and their weaknesses.

CS:GO/Quake has no RNG in methods, but RNG in map or tactics. Sure "The best man wins" and several rounds will prove that... But if, mathematically, two teams were 100% even in skill it'd basically become a coinflip of who had the luck of seeing the enemy first. Or walk into the right/wrong room at the right/wrong time.

Games like Dota2 is almost like this, but here map awareness/RNG is further explored by the addition of wards: Ways to see the map; This becomes a part of the overall strategy, just like team-movements in CS:GO. Any good player will tell you that wards and map-vision is probably one, if not the, biggest factor in a game like Dota2.

Now on this, there's further random factors in games like Dota2. "Runes" spawn at fixed times, but with random effects. Getting a "Double Damage" run on a hero who has a passive skill that makes her crits do 400% damage, basically makes her crits now do 800% damage, which you can understand is a insane thing to get. Then there's "Haste", which will let any strong early-game hero pretty much chase down any hurt hero on the map.

These are non-skill effected random factors. Skill will affect how much you get out of them, but they are also "unfair" in that sense that skill aside; There's "Good" runes and "Bad" runes.

A RNG factor I am not fully sure I like, tho I have no better solution, is the random spawntime of the "big boss" in dota2 (Roshan) who drops the Aegis (if you die, respawn at exact location of death after 3 sec with full hp/mana). Lategame this Aegis, and the "roshpit" is a heavily contested zone. Frankly its THE most contested zone lategame inbetween two teams evenly matched.

The problem is, Roshan respawns in random intervals after his death. Sure there's a "minimum" and "max" time, but its still random.

In some cases, a team that's barely losing manages to get several quick rosh respawns in a row, only to have the winning team get nearly none due to long respawns when they check the roshpit.

Now. The downside of RNG is that it takes away from the achievement of the victor, it basically makes it so the "lesser" part has a chance. But that's also why RNG is liked in so many cases; The upside of RNG is that it makes the game unpredictable, exciting, it makes it so that the underdog has a chance and "anything can happen".

If all Dota/LoL games had the map shown constantly, well the team with the best team-fighting would win. There's be no real contest, no real battle of wits, deceit, albeit there'd still be feints and such (like say, in Boxing, even when all factors are known) but it wouldn't be the same. It wouldn't be RNG, it'd be pure sleight of hand, reactions and skill.

I've never really liked RNG most of the time, frankly it has nothing to ever do in a shooter: Which is why I fucking detest random-crits in TF2; Totally nullifying skill, nearly instantly killing whoever is in the way (or several if you're a soldier).

Tho, in the mean time it also adds in some games. In games like Dota2 where there's already so many random factors (pathing, creeps, spawns, runes, rosh, crits, misses etc) it adds to it, it adds to the chaos but under a controlled manner so to speak. It makes for spectacular situations where casters or players can scream as the last two heroes alive battle eachother, missing hit after hit, both crawling at the edge of death yet no victor is clear: Because there's the factor of RNG; The battle isn't over till it is over.

So,. in short, sometimes its good, sometimes its bad, sometimes its a factor of the medium, and sometimes its pure RNG.