So I sold my Wii U, moving to Ireland. Had the guy come over, add a new nnid and account. Then I deleted all of my accounts from the system. Will his family be ok to use my content? We logged in as him and all the games were still on the system. If this works, why don't others do it? Or is there an issue? 

He got a good deal. $250 with 12 games, a pro controller and luigi wiimote. I kept mk8 to sell separately.

Hm, it seems like your NNID is still on the system. If you create a new NNID you still have access to all games purchased on other NNIDs.

How did you remove your account? You shouldn't delete the NNID, only the user. Games are attached to the NNID.

Content might stay on system till you try to add your account on another system. How they handle your account is weird but there might be no issue with those games being on that system till you contact customer service to try and get your games back on a new system. The way that they handle your account is like a black hole. I believe talking to customer service is the only way to get your old games available to download onto a new system.

Actually, I have no fucking clue what I'm talking about because I've never sold or lost a system.

You should system transfer your Wii U before you sell it

To my knowledge that stuff is hardware locked. Deleting the NNID off the WiiU used to just destroy your ID forever

I will need a new nnid in a new region anyway.

this was my concern. He didn't seem to mind either way, so if the games don't work it's no biggie I guess but Im curious to know. 

Good to hear you've found a buyer. Let us know when you get a new console.