It's a tough question.  It's hard to say how much work would be saved by customizing UE4 vs customizing in house tech.  Cause they would want to customize it, for both performance and aesthetic reasons.  But I do think some teams could make use of it.  The Pokémon team at GameFreak for example aren't wizards with engine design, I could see a full HD Pokémon being more feasible with UE4.  Though we can't be sure that, say, the Breath of the Wild engine isn't just as easy to work with since we've never seen it.  

Well regardless of how well and easy it is to port over third party games, the true question is if they will actually do it. Will it be worth their investment? Because the thing is, we've already heard how easy and well third parties can do this (regardless of the naysayers). We will see in the near future, I hope this system leads to a turnaround in Nintendo's fortune and relative image in the eyes of "gamers". Even with proof we're still hearing people say dumb stuff like "it's very weak" and the classic "the hardware is outdated".

A lot more games use Unreal than I would have thought so it's good to see