Hi there,
We announced a lot of information on UMG in our twitch stream which you can see here. Twitch
Short version is that it will support everything you want and will also include 3D support.
We chose to use Slate as the backing for UMG because we already have a huge library of common widgets (such as robust list/tree support, editable text blocks, menu system, combo boxes, etc) which will be really hard to replicate in canvas without a lot of work and we didnt want to duplicate that effort on our side. Additionally Slate supports efficient batching to reduce draw overhead and canvas is lacking in that department. We’d be happy to hear your feedback on our plans for UMG. We are pretty open to sharing all the details so just ask.