Riven requires some skill to spell weave and the what not, alongside positioning and timing because of her relative squish without her e up.
That being said, she has a pretty high return on effort. Because everything scales with AD, she experiences large spikes in power for small increases in competency.
Also, new cassio is hella great. Guaranteed 100 stacks of passive at 10 minutes by natural stacking, probably closer to 200 by 10 since it's so easy to land that q. Easy 500 by 25 minutes no problems. And her E feels so smooth to use now, it's just wonderful.