From people who actually plays Yasuo, Vayne, and Riven?
Even Riot Zirene (Diamond 1) once commentated in an LCS game that Vayne does not win any lane until around 11 minutes when she gets BoTRK.
Granted, there are champions that Vayne and Yasuo counters.
For them, the lane is pretty much hell because these champions can't abuse Vayne or Yasuo's early games.
To better explain why they are weak early and Riven late, I will elaborate on my points.
Yasuo starts off with low base health, slightly above 500, low base armor, around 24, and flat 30 MR.
He has one of the worse starts of any champion to compensate for his crazy 2x critical chance.
If it were not for the AD masteries and offensive runes Yasuo takes, most mid lane mages can literally win if they straight up AA each other.
At level 1, most Yasuo players start Q. Others start E, but assuming that this Yasuo has taken Q, he cannot trade effectively at level 1.
After the first minion wave, AA him if you haven't already to pop his 13 second + CD shield, and use your abilities. If he tries to walk up to you to trade back, AA him some more.
If he took E, do the same thing. Don't stand near your minions and he can't get to you. He does less damage with E, but is more mobile.
At level 2, he either took E or Q, repeat above.
At level 3, just watch for his wind wall. Yasuo might try to E to you and throw out a wind wall to block your CCs. However, he can't kill you in one combo, and W has a ~25 second CD.
At level 6, just watch for his Tornado and knock ups from his Jungler. Don't stay in lane if you are below 35% health. If he gets a knock up, the ultimate will kill you.
The key to playing Yasuo is to take initiative. If you don't pop his shield and he E's on to you, he wins the trade.
Vayne on the other hand, starts off with mediocre ADC stats, and have pretty good AS growth.
Her level 1 is not bad, and her level 2 is strong too if she hits it before her opponent.
However, things go down hill for her after level 1/2. She has a 550 AA range. Most champions either out ranges her (Jinx, Caitlyn, etc.) or have some type of damage ability that can out trade her (Graves, Corki, Sivir). Essentially, she shouldn't win trades.
She hits a spike at level 6. However, since Vayne doesn't level Tumble first, her Q is on a ~8 second CD. She still can't beat most ADCs such as Graves, Corki, or Ezreal. Vayne only really starts to shine when she hits level 11 and gets her ~2 second tumbles and BoTRK.
Riven is a strong duelist at all stages of the game. I believe she can do upwards of 810% AD in one combo, and that is not including her shield and stun.
However, she is somewhat weak in team fights. In a large fight, one second of CC might spell death. In a 1 v 1 scenario, most champions with CCs don't have damage, and those that have damage have no or conditional CCs. All of Riven's survivability is banked on her shield. If she gets rooted or stunned and cannot use her shield, she drops extremely quickly.
Secondly. she cannot engage on an enemy without using her offensive abilities. Riven have low CDs, but Broken Wings and Valor can't cover the distance quickly like a Talon or Katarina blink. She needs the enemies to come to her and for them to not focus their CCs on her to be useful.
that make bad early games redundant because they give so much safety. So to recap, her early game is extremely safe and not nearly as bad as her late game warrants, and she's got a support to add more safety to make sure she reaches that unstoppable late game.
: My Empowered autoattack is not a projectile. (Does more damage than the shield holds after getting a Sheen)
: I hope you like some air conditioning because it's about to get cold. (No need to aim. Your projectile is on low CD and the more the guy zips around, the more damage he takes from that zone of blizzards)
: I'm sorry. What's this about... 'projectiles' that I hear? (Granted can't aim for shit but if he runs out, W and slow him for life)
: Space aids. DoTs for life.
: Click and shred.
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