I've never found Ashiok remotely "too good", and imho that's because he was pretty good in standard - but Cube isn't standard.
I like him a lot because his +ability feeds into mill strategies and is great for a control vs control matchup, whereas his -X ability is super fun. I've never seen the -10 happen, and it isn't even win-the-game good by itself, so eh.
Mostly, though, he works as a trap - for your opponent. "Can I kill my opponent before he summons one of my cool thingies in a few turns, or do I go for the planeswalker first?" That's part of what makes him so good - he's bait, but he has a lot of potential to do cool stuff if left unchecked. If he was monocoloured and had 5 loyalty first-turn, I'd be more concerned I guess, but being multicolour, I don't mind at all.
Elesh Norn is another can of biscuits. I love her as a top-end in white for any deck; control, midrange, reanimator, she's a boss-ass bitch. She tends to be a mini-wrath effect when she comes down, and she can lock down a win, which is noteworthy. But at 7 mana, and for how FRIGGIN COOL SHE IS, we tolerate that, because removal does exist and players should be punished (imho) for not A) drafting it, or B) building a deck that can win versus big win-beasts like Miss Norn. But that's just our perspective over here - YPMV. Every now and then we go "wait, so... she's too strong, right?" but then we ultimately go "....but let's keep her, she's awesome".