No CC reduction/nullifiers? Gl with ulti bro.

You don't build a banshee's veil, which in itself is a mistake, but your explanation of it puts it over the edge. "any team with a brain" will use expendable CC on you and then use hard CC to stop you, that's not how it works.

Without a banshee's veil:
Sion stuns you right after you ult. 0.5 sec ult.

With a banshee's veil:
Even a coordinated team will take quite a bit of time to get mundo's cleaver to pop your shield THEN sion to stun. 1.5 - 3 sec ult, much more damage, much more slow/AS debuff, much more team help.

Even if the team throws every CC imaginable at you in that same instant, giving you a 0.5 second ult, that's another CC wasted on you, not your team. You're Nunu: you can do damage, but it's not that much compared to Ashe or Tristana. You're a bursty support, meant to keep constant buffs on your carry and constant debuffs on their carry, and occasionally a heavy AoE nuke that will force the team to focus you or split up/suffer heavy damage. That banshee's veil is essential for this role.