The reason I chose the examples I gave is because of generally accepted strengths and counters in LoL.
Generally speaking, an assassin will be able to oneshot/nearly oneshot an adc level ~8 onwards for the majority of the game, until the adc buys defensive items, and even then there's a good chance. As an ADC, I can avoid confrontation with an assassin or kill him if he's not powerful enough to dual me/I've gotten an advantage in lane, but that's situational. In most cases, assassin VS ADC 1v1 the assassin will kill the ADC (Or pop GA if ADC has one).
Generally speaking, an ADC will be able to kill a bruiser in a 1v1 due to ADCs being able to kite and most adcs used having some form of mobility nowadays. Sometimes bruisers will be able to annihilate ADCs, like irelia VS ashe, but in most cases an ADC has an advantage when combating a bruiser.
The difference between these two cases I have already outlined. It's not a question of equal roles facing each other, it's a question of equal counters facing each other. If we use an oversimplified example, rock paper scissors, then the way I would compare it is in this, rock can immediately crash onto scissors once and destroy them. But scissors need to cut up paper a lot, it takes some time, and a considerable more mechanical skill is involved than letting a rock drop on a pair of scissors.
I realize that my examples are oversimplified, but in this situation we HAVE to oversimplify things if we are to make general claims. Everything ever is situational in league.
One last thing I want to point out is that you used the words "late game". ADCs have their late game fantasy, and I'll admit that for some ADCs the amount of damage they output at that point is also comparatively high.
BUT the thing is assassins can usually one-combo an adc by level 8 onwards, as I've already said, especially if the assassin gets a kill or 3, even if the adc did laning perfectly and is relatively ahead. They'll be behind on levels compared to the assassin in mid/top lane as well. ADCs need to deal with assassins being annoying and oneshotting them with (sometimes) point and click abilities from like, 10-12 minutes onwards, give or take. A game usually lasts for over 25 minutes at LEAST. ADCs are threatening and deal ridiculous amounts of damage after they get their second item if they're ahead, or third/fourth item if they're behind. On average ADCs reach the point where they're scary far later, the enemy team has to deal with a powerful adc opponent for a (relatively) shorter time than the adc has to deal with the annoying point and click assassin that will oneshot him if he comes near.
Sorry for the wall of text.