This is a long article from AdWeek that looks at mobile advertising as a whole, weighing up the pros and cons of it depending on what brand/product you are peddling.
As the article points out, mobile is a very personal screen. It's intended to be useful, first and foremost; a communication link with loved ones and colleagues, an encyclopedia, a tool to help in our everyday. Anything that doesn't provide value is at best ignored… or at worst despised and therefore damaging to your brand.
The conclusion to all this comes at the end of AdWeek's piece: "While a Facebook or a Google or even a consumer can be mobile-first, it might not make much sense for a marketer to be."
So that's not to say no brands should be mobile-first, but that if they are, they should know why.