The concept is quite 'nice' as isolated images, but I find that there are simply too many assumptions being made and real-world usage being ignored.
Sorry for the wall of text.
"UIKit for macOS"
The dock being transparent and without any framing is visually 'undocked'. This is confusing for users because now the dock appears to be on the same level as the desktop when it is supposed to be always on top. The mock-up conveniently ignores the fact that practically everyone has icons on their desktop. So what differentiates them?
The screenshots give off an impression of 'clean' because the apps shown are mostly content-consumption apps like Netflix, News, etc. However the fundamental difference between iOS and OS X has been that iOS gravitates towards content-consumption while OS X easily accommodates traditional workflows for content-creation like audio, video, image editing and so on.
In apps that exists in both iOS and OS X, there are still differences in how we work with them on each platform. In Mail, I am likely to drag-and-drop mail threads across mailboxes & email accounts. I am likely to jump between folders in a single click. How do I do that in that mockup?
Also I'm not sure if such an implementation of Inbox will work on 15" rMBPs up to 27" iMacs. One of the biggest complaint about the current resolution-independence implementation in iOS has been that apps are lazily scaled for the iPad, creating huge white-spaces and wide paragraphs.
For Music, splitting iTunes seems to be the most obvious solution, but why is something so obvious not being implemented immediately by Apple? Where does iTunes Store go? How do I sync my iOS devices? How do I manage ringtones? How do I differentiate between purchases, streams, iTunes Matched and uploaded tracks? How do I edit the track information?
For Pages (a content-creation app), look at how apart from the theoretical collaborative features the rest of the interface just got flattened. How does the grey-on-grey toggles at the side accommodate Accessibility (poor vision, etc.), one of the biggest complaints of iOS 7?
The Filesystem
I'm hesitant about such an oversimplification of the filesystem because in real-world use case you can have different files of the same name in different folders. Things like
index.html and
style.css for the 20 different concepts for the same website you are working on. You have files that you don't remember naming that way, like
thisisshit.psd for that sketch you are rejecting and
YEAHWOOHOO.ai for that illustration you completed and is now looking for a few days later.
Basically, the mockups in their current stage gives me no compelling reason to own both an iPad and a Mac.