Okay, so I'm fairly new to Ubers and I had a few questions relating to the viability list. These are actual questions, as in, I'm out of the loop and I want to know the answers, not arguments to change anything. (I also had some more questions below, sorry for so many, but I figure I may as well condense them into one post. Feel free to just answer some rather than all of them if you'd like.)
1) Why is Mewtwo S Rank? Is it basically the Charizard effect in OU where you have no idea whether it will go Physical or Special? Also, which Mewtwo (X, Y, or normal with LO or whatever) is generally considered superior (like Charizard X right now is generally considered slightly superior in OU, despite it playing a different role).
2) Why is Chansey outclassed by Blissey in Ubers? Keep in mind I'm coming from OU, where everyone freaks out at you if you use Blissey over Eviolite Chansey. I don't really see knock off that much in Ubers (except one time from a Blaziken).
3) I know why Arceus-Normal is top ranked (E-Killer), but why Arceus-Ghost? I've been using him and he's done pretty well for me, but not necessarily brilliant.
Qs unrelated to viability list:
1) What do Dialga and Palkia each like to do?
2) Is CM Mega Mewtwo Y viable? If not, what would he rather be doing?
Thanks!!