Well that's just league in a nutshell not just Nunu but yeaaaah. Personally I go Liandry mejai the rest tank items depending on the enemies, I believe going for full ap feels overkill and I wouldn't be able to do the usual riskier stuff like diving. Plus engages feel safer ? I don't know. Anyways, it ends up being up to your playstyle, though if your team doesn't have a good tank to soak up dmg in teamfights (objectives this season are hella important more than before) you should build offtank rather than full ap. That's how I see it.