or, instead off geting a nice execute on aoe, you can't reach enemies for a couple of pasive procs and your ult deals piss-damage anyway.
it's not that easy to get off those passive procs, especially in a teamfight.
The two strongest situation for riven currently are bursting squishys 1v1 vi animation canceling, which is nerfed rather than buffed as she won't have 3 passivestacks after all is done, but 1, and her ult will be less potent as an execute, since she won't have more than 2-3 passive stacks in that time.
Another situation often seen is a riven dashing in with 3 fast Qs and a ult for cleanup, which again deals less dmage now as you won't have blade stacks...
I'm not a riven main, nor do i really like faceing the current riven, but i have enough experience on and vs her to see that this is overall a nerf, and everyone who does think it'S a buff probably nerver really played her.
Not saying it's a huge nerf, but certainly not a buff.