the term 'hard to play' or 'skill intensive' can both be incredibly subjective: riven has combo's that people can learn to improve their performance on the champion, just like she has timings that people can practise in order to get the most out of her.
for the rest its all knowing your timings, match-ups and build paths. Just like any other champion the problem with riven lies within the way her kit interacts in general: she can abuse certain runes to an incredible degree, and her trading pattern in lane looks more like that of renekton: uninteractive and quick / decisive.
this is not even going into her numbers: riven, for what she is supposed to do (early lane bully, mid game champ or late game? i dont know) she is too good at it. her somewhat ambiguous nature when it comes her position in the game (top laner, but good at any stage unless severely behind) and flexible kit makes her problematic.
for a champ released in season 1 riven has a HUGE number of tools at her disposal in order to make plays, escape, trade safely, all-in and just about anything. not only that, they only have 1 condition: are they off cooldown? since riven has no resource she is allowed to spam her abilities as much as she likes. as long as she build CDR she'll be fine.
other champions who share this nature however are far less problematic, why? because their cooldowns are read far more easily and they function a lot more simply. take Garen and renekton for example. if they start moving up to you, you know what they are gonna do and HOW they are going to do it.
The same goes for riven but her flexibility makes her far more unpredictable: she could 1st Q in to farm minions and instantly E out, or continue by using E to reposition her and then stun you with her W and proceed to use her remaining Q cast to decimate you.
all in all this champion is a general design and balance mess. riot should make up their mind whether she is early/mid/late strong and adjust her power accordingly or keep her neutral/good at all stages but reduce her overall power.
unfortunately riven has a massive fan-base of OTP's and other riven mains, so riot probably wont do too much about it until her numbers really start to get out of control (like 60% WR)
That's all