I'm always skeptical when I hear the "mobile-first!" battlecry in UX design circles. While I recognize that apps and mobile sites must have a delightful consistent user experience, I'll never design a desktop app based on it's ability to work on a smart phone. There has to be a dedicated website design, and a dedicated mobile browser/app design to truly deliver a great user experience across hardware devices.
I've seen desktop designs suffer because companies insist on using one code base for all devices, thinking that making a responsive/adaptive browser experience for their app is acceptable across formats. The example I'm thinking of is when white space is forced on a desktop design because the developers insisted on making the browser view collapse to a mobile stack. Essentially the desktop views of the app suffered in because there is a full column of wasted white space when it could have been used to enhance the desktop experience.
In the example above, the app forces the user to enter lots of data in the small blue area to the far right of the desktop wide browser. There are numerous fields that are collapsed via an accordion controls make even more work to fill out the form. Sure, this makes the app response - you can narrow the width of the browser to mobile phone size and everything fits. But wouldn't a dedicated desktop experience be better? Especially in the case of forms that will typically never be used on a mobile platform. Meanwhile the left side of the app creates a file name list - look at all the space it uses - unless your filenames are incredibly long it is again more wasted space.
I was happy to see that the very app I use to create this blog (wix.com) only generates two views - mobile or desktop - and it makes sense. I'm not going to be tweaking this site on my phone anywhere, ever. Sure, I can type up a post and mail it to the site like most, but I'm not going to sit down on a bus and say "gotta whip that section of the site together here on my iPhone." At Wix they realized that you really only need to spawn code for two form factors - mobile or desktop. The Desktop view takes care of the tablets by setting the min width at 768 so it will work on either portrait or landscape, and mobile view covers everything below that breakpoint. Doing more breakpoints means keeping track of a lot more UI as well, especially if the customers want distinctive views for phone, tablet, and desktop.
Most web apps can provide the customer the interface they need within a maximum width of 1024 pixels, which is the same as a landscape view for an older tablet. Again, copy/cutting and pasting, and the dozens of other microinteractions we do on our laptops and desktops with a trackpad or a mouse is much more painful on a tablet (with some exceptionally well devised note takers), so people tend to use a larger format.