Building 3 websites versus 1 responsive.
Google championed for responsive or rather mobile friendly websites versus having two sites, one for mobile and the other for desktop. Granted having one website made search much more organized. In the past when searching for a site, it was sometimes a gamble in that you could land on the mobile URL or the desktop URL.
The concept of mobile first is problematic. First and foremost, there is no robust platform that allows you to create an entire website on a mobile device. Which would be ideal to view a live version of the site while still in development.
The other issue is the CTAs versus the viewports. Often times items have to be rearranged accordingly to maximize impact from a UX perspective. Hence the need to build 3 websites (desktop, tablet, and mobile). Given that users still use desktops at work, and its the only way to efficiently maximize website real estate. In which it makes all the difference when it comes to CTAs.