What are the disadvantages of purchasing pre-owned games for the Wii U?

The only new game I bought for my Wii U was Splatoon. I haven't noticed anything wrong with any of my other (used) games. I won't list them, but none of them require any sort of method that tie it to the original console. So I can't speak for every Wii U game released, but I haven't run into this issue with about 10 or so games.
The only possible issue I could see, is if a game had online components that were locked until you bought a online pass. Some (very few) PS4 and XBONE titles do this unfortunately (Resistance 3 comes to mind), but I have yet to see this on any Wii U games I own. And this would be a decision of the game developer/publisher (not sure who would make that call?) and not of Nintendo in general, so you couldn't generalize for all used Wii U games.

Like Robotnik commented, if you do happen to find one that you can't play for that reason, return it. They have no business selling you something you can't use.