Youmuu's active is incredibly good? It gives movement speed. It's like a temporary +70 movement speed(Using 325 move speed champion with basic boots as an example) for 6 seconds. Mobis gives 115 for an entire duration that you're not attacked. How is Youmuu's active "incredible"? It isn't, it's an okay active but not game changing like Zhonyas. Edge of Night is a pretty good active, it's a banshees veil on command, thing is it requires a channel time and is only used for engages, not for your team to catch up when you Morgana ult and stun an entire team while being invulnerable, or Fiddle ult and wither the enemy down to nothing, again while being invulnerable. It's for a basic engage(and you don't even get away in most situations if you do engage with it), or a way to dodge an ability if you're running away. Not saying Edge of Night isn't a good active, but it's not going to give you time to wait for your cooldowns after engaging, and it's not going to give you the safety of waiting for your team to follow up either.
Zhonyas gives CDR, armor(more than EoN's MR), AP, and it's active. EoN gives AD, MR, Lethality, OoC movement speed, and it's active. If you add Lethality on top of the AD it's 75 AD, and that's with a level 18 enemy. That's 5 more AD(again at level 18) than AP on Zhonyas, and 10 less MR than armor from Zhonyas. I'd say they're pretty similar, aside from the fact that instead of CDR, a stat that is infinitely better than OoC movement speed, EoN gets the latter.