if there are no game updates (and there really haven't been... for anything) then its pretty safe. there is a theoretical possibility that they could issue a game update to try to find mocha/rednand/etc hooks, almost all of them are covered up to cloak themselves against detectors but its theoretically possible. the absolute worst that could happen though is you could get banned for online play for that game, that HAS happened to people who have used cheat hacks in SSMB. There is no mechanism in place to brick Wii U's remotely or stop you from playing installed games offline.
i play MK8 online all the time but I boot into system firmware usually when doing it. When you set up rednand just don't turn on autoboot, then every time you start your wii u you either can run system firmware (100% legit wii u), haxchi, mocha, or vWii from the startup boot menu.
remember this hack stuff has been out for a year now, nintendo could have issued a patch at any time to prevent updated/new wii u's from doing it but they have not cared at all. They DID issue a patch for the 3DS about 2 months after it was jail broken to prevent anymore, so they still really care about that platform.