I chose not to include Yasuo mid for a couple reasons.
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Laziness. I'd have to combine both sets of data after mathing out the difference in his play rate across lanes.
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Comparing his performance in the same lane as her gives a more direct and accurate comparison.
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I just think he's better top lane, for similar reasons to Riven; the lane is longer so your all in is more effective.
To be honest I play neither champion, but from what I hear and just looking at their kits Yasuo seems harder. Due to Riven's on demand shield, knock up, aoe stun and ranged execute she has a lot of tools to deal with most matchups and can simply blow people up. Yasuo has to time his Wind Wall, and land skill shots.
To be fair, Riven's Q and E turn herself into a skill shot. :p I have only played Yasuo a handful of times, but my performance was no worse than my early Riven games. Even so, I'd consider my own experience to be anecdotal, the statistics imply their skill floor is similar, whose is higher is arguable, Riven takes longer to hit the median, but her median is higher.
Yasuo's improvement curve if nothing else is less linear than Riven's, the improvement a Yasuo sees in those 70ish games is less than the improvement Riven sees in the same time, so in that sense he could be harder.
In terms of who is harder to execute at the highest level, I'd have to actually get there, and frankly, I have a job, a life, and only decent reaction time.