Node.js backend, using Express.js and a REST API framework we built ourselves based on it
Frontend app in Ember.js and a couple others in React/Redux
Whole thing is hosted on Heroku.
When we first started with Node.js, it was pretty bleeding edge to run as a production app, and we started with just a small microservice. It worked beautifully, and we found that our "silos" of frontenders and backenders was really helped. Everyone could review at least all the code in javascript.
Debugging can be annoying on production, but having run Ruby and PHP products before, we found it no worse (but not better).
A lot of people find it uncomfortable how fast the ecosystem moves but we don't really mind. It's kind of just the way it is. Just the right fit for our company since we as a culture like more recent things.
Although as well, I want to say, Node.js and javascript in general has grown in utility and functionality by leaps and bounds in a few years, not to mention the sugar and developer happiness.
I would really recommend Node.js in a small startup especially where small dev teams need to work closer together and really understand what each other is doing.