Alistar's WQ combo is a lot easier to both figure out and learn (it's literally just holding Q after using W).
And more importantly, it's generally a weaker combo than managing to get close by other means and locking them down with Q as you reposition to knock them back with W.
And it doesn't compress how quickly he can use the abilities in sequence (at least not much), only where the target is.
Alistar's combo is also a lot less disruptive of his playstyle, it gives him better engage, something he's already good at, and he sacrifices part of his ability to disrupt the enemy formation in the form of a knockback to do so.
Riven's animation cancelling (at least from what I've heard on the forums, and minorly seen on youtube) disrupts her entire playstyle, turning her from a Sustained DPS Bruiser to a Bursty, Tanky, Assassin.
Alistar still has to play around his limitations when doing his WQ combo, and trades away his peel for the engage he gets. He's still gated as hard by cooldowns and mana as he is when he uses his skills more effectively to knock up an opponent and then reposition to knoeck them into his tower.
Riven has no mana, and is only gated by Cooldowns which scales lower and lower as the game goes on and she builds more CDR and ranks up her W and E.
She is of course allowed to be resourceless and only lightly gated by cooldowns because her kit takes time to unload.
You wouldn't give a 4 sec CD AD scaling resourceless shield which empowers your next attack, to a bursty assassin, but on a sustained dps bruiser it fits well.
Except when she can use a secret trick to unload it in a 4th of the time it usually takes.
Riven is two different champions depending on whether the player knows how to excecute a secret trick or not.
A lot of other champions can be played differently, and can be made to fit drastically different roles. This is a good thing, but other champions do it with builds. Runes, masteries, skill orders items. And some champions can seem totally different in the hands of someone who knows the champion well, and has the right insight in how to use them to their full effect in some weird off-build.
They're played differently, and the opponent can react to whether you're going AP or AD or Blue Ezreal when they see what items you buy.
You can't react to Riven suddenly pressing all her burst into a short window with no penalty to her decision. (And again, while you can't react to Alistar's W suddenly ending with a Q, he at least gives up his knockback to do so, meaning he's shortened his CC on you and can't use it to peel or knock you back into his team/tower)
And you can't know whether Riven's gonna swing at you for an extended period of time, using her mobility to jump around and simply outplay you, or if she's just gonna burst you before you can say "flash or die".
But IMO the worst part of it is that without almost digging up the info (or atleast catching on to the debate on the subject here on Boards/Reddit/whatever), you don't even know that you can use an exploit to severely alter the nature of the champion. I know I wouldn't have gotten Riven if I thought I had no chance to ever learn her properly.
I find it especially disconcerting that Riven is (IIRC) in the pre-6 rotation on at least EUNE. Because a champion semi-reliant on a secret exploit to be effective is definatively intuitive and forgiving to play as...