Yeah, that's fair. I dunno, I can't really hear anything wrong with Fluttershy or Applejack no matter how hard I try, but your ears are probably much better than mine :p
I'm not too sure about that. From my testing, Fluttershy, Applejack, Twilight, Chrysalis, Rise, and Kyu (and a bunch of other characters that I'm surely forgetting) seem to perform phenomenally well. Especially Chrysalis, her emotions are extremely believable, and Fluttershy/Applejack/Rise/Kyu have almost zero noise for every generation. This might be the most impressive site I've ever seen.
Oh, I somehow forgot all of the TF2 characters. Some of them do struggle (Medic the most, I think) but everyone else seems incredibly good.
And the Daria characters, too. Honestly, the vast majority of characters are already near-perfect.
I've been seeing quite a few skits being posted on /r/tf2 (https://www.reddit.com/r/tf2/comments/kr374q/honestly_idk_i_...) and all of the voices sound pretty much perfect to me. But as you said, it's subjective.