5/0 Nunu vs 0/3 Yi with half a brain cell. Who wins?
He ignores E slow with his R. Q's to dodge willump's turn, so running away is impossible. Think you can W away to safety? He can just outrun that and continue slapping your butt if he's half decent at attack move. He Outplays R with Meditate. All while shredding through all 4 of your armor items with his true damage. Thus far, there's nothing that nunu can feasibly build to deal with a Yi. It doesn't matter if he's 10/0 or 0/3. If he has his Bloodrazer and his rageblade, he kills you regardless of how fed you are.
My only success vs Yi thus far has been getting my team ahead and warding my entrances, and praying that they rotate in time to save my butt before Yi rips me to shreds. Unfortunately, solo queue is solo queue, and sometimes the mid that you spent several minutes camping refuses to rotate. Without assistance from teammates, you literally do not have a jungle against this champion.
Anyways salty rant over. Nunu is actually a solid champion overall, but this has got to be hands down, one of the most one sided matchups in current LoL.
who wrecks Yi’s shit like a high-fiber diet.
and 
for example can jump over a wall and escape. 
can straight up kill Yi if he's ahead. Failing that he has the option to ult or Q away to a nearby camp. 
not a super common jungler, but Yi isn't going to appreciate getting Brittle proc'd and beat down by a fed goat. Nunu's main two outs are his W, which isn't enough movespeed to escape, and his E, which Yi can ignore in its entirety. It's a very rough matchup for the guy, moreso than any other tank. You lack the option to fight or run away.