The Switch seems to be a clean break from previous Nintendo consoles, the upcoming system not backwards compatible with old games or controllers. However, this might not always be the case. In an interview with Time, Kimishima revealed that they were considering making certain old controllers usable with a system update. This is certainly interesting, controllers like the Wii Remote being a Nintendo staple for over 10 years now. Check out the full interview here.
Is Switch powerful enough to emulate Wii U games, or would they have to be ports, like Mario Kart 8 Deluxe?
We can take games and bring them and make them playable on Switch. So they can be remade for Switch, yes.
That said, Switch is not backward compatible with games designed for other systems, and is not currently compatible with controllers designed for other systems. Support for certain controllers may be considered for a future update. In some cases, games from past systems may be re-released for the Nintendo Switch system as either enhanced or original versions.