Nunu and durability in general

Anyone else feel like durability is far worse than it should be? Using nunu as an example, my friends and i were discussing this set and how some characters look great on paper but in play feel AWFUL. Cass and Nunu being the stars of the conversation. For a 3cost tank, nunu feels... really weak. 50% durability is SUPPOSED to be 50% reduced damag, but even with his spell going and 200+arm/MR with the enemy having no shred... he melts. So fast.

Cass feels like watching paint dry, her spell is lackluster for a 'carry' option.

and generally speaking durability doesnt seem to do what it says on the tin. 50% durability doesnt feel like it cuts damage in half... Its made to sound like its the tank version of true damage, but it just... doesnt.