My agency spent a while planning a Node.js API CMS build of our own, as well as evaluating Prismic and Contentful. We needed something we could confidently sell to clients, that was open source, user friendly and easy to extend. It also had to run on our clients' own stacks and meet their audit criteria. Wagtail suits those requirements well.
It's worth noting that the Wagtail API module is for GETting content at this point – though I hear a more fully featured read/write API is in the works. Wagtail "is just Django" so it's easy enough to plug in the REST framework and build endpoints on that.
I implemented Prismic for a startup recently, and the API is also read-only. It's got some very powerful semantic content markup and versioning stuff, but I found it quite a chore to get more than single nodes out of it. In the end, our content editors didn't like the git-centric "content releases" metaphor, so they moved off to something else.
Contentful has the read/write API thing down, good support, very actively developed. Unfortunately it doesn't support tree-based content, preferring flat collections with relations, though trees are something many users are asking for. As we're migrating lots of sites (off a PHP CMS) that have well-tested trees, we couldn't quite go that far just yet.
tl;dr? No solution is perfect, but Wagtail is making us happy devs, and the community is very friendly. Good luck with your CMS hunt!