How much have Keystones had an impact?

Bit of a broad question there, so let me narrow down what I'm shooting for here. Although I'm not keen on Keystones, this isn't a salt-circlejerk thread in the making, I'm genuinely curious about this.

I was a wee greenhorn in Season 5; and during the time Keystones were released, I was off on traveling jobs for months and didn't get to brush up on League the way I do today. With the introduction (and learning) of Keystones, were any champions' "niches" changed? Did anyone's roles get twisted around? Did any champions change in such a way that was unhealthy, in your opinion?

Enlighten me, pls!