I personally make decisions based on what the project requires rather than some template. Most websites for me are informational, so mobile-first works well. Not a whole lot of functionality to test so we can focus on building the website and make it look on any size device.

However, apps are a different ball game. I'd rather focus on usability, experience, and functionality. I'm not going to try and hobble the app by trying to beat a round peg into a triangle, square, whatever shaped hole. I find that most web applications which have large swathes of functionality benefit from native code, so I usually skip mobile-first design and just do a native app (Xamarin or Titanium works). Why? Because I don't want to sacrifice usability, experience, and functionality. Most apps, I've found out, have different experience, usability, and functionality on difference devices.