Wow, this seems like a cool product. I can't imagine building a product that relies so heavily on the UI of another service. How are updates handled?
Being only usable in Chrome is also a disadvantage of zenhub in my opinion.
huboard and waffle are much more tightly integrated with github from what I saw, to the extent that all data lives in github, they just display it on a kanban board (as far as I could tell).