This will probably be my first AND last post on the topic.
It's just an engine, nothing more! The matter to be concerned about is the possibility that some software may exploit that engine to a degree that overwhelms the Wii U, to the point where scalling back certain details makes it look/feel like an entirely different game.
Because of the deminishing returns, I personally believe that hardware in the near future will have to be tremendously more powerful than Wii U for the above case to be possible. The famed 3-4x multiple just won't cut it. It's still a valid concern in some - albeit less major - ways.
In any event, it's really up to the developers whether or not they want their UE4 game to look like the original Pac Man, or the industry's next best-looking game - and everything in between.
For reasons stated above, the power of the Wii U - in relations to UE4 - might only matter on a per game basis. Whether the engine is eventually supported on Wii U, will come down to the feature set of the console(devs already stated it has modern features), and how agressively Epic is trying to push their new engine. IIRC, they already implied that they plan to support other platforms much sooner than they did with UE3 and it's eventual debut on mobile, so we'll see IF and where Wii U fits, in their forecast.
As it stands now, Epic may be in a tough situation. Even IF they plan to support Wii U, an early confirmation, puts them in a less favourable position to lobby console manufacturers(Sony, MS) to overcompensate on the performance of their upcoming systems. Just imagine...
MS & Sony: Why should we push for more power (= higher manufacturing cost), if we can already outperform Wii U which supports your UE4? To gain parity with PCs? The same PCs which will see more powerful GPUs every 6 freaking months? Really now, Epic!
That scenario wouldn't bode well for Epic, so for now, it seems like rather than just OFFICIALLY say "No Wii U"... they are doing a bit of damage-control with the whole "not announced for platforms beyond PC" statement. If you ask me, it seems like they are open to the possibility of future Wii U support.
To reiterate(for those who like to quote a sentence while ignoring content/context): UE4 support for Wii U would not surprise me at all - it's just an engine, afterall. It's the individual games and how far devs decide to exploit the engine that should be more of a concern.