In terms of usability, monochrome line icons are not particularly a good idea. At a glance they tend to look the same. They’re often abstracted to the point where you need to be familiar with the “vernacular” of app iconography to interpret them.
They are mostly used for aesthetic reasons. It wasn’t that long ago that icons were expected to use color and shading. For usability’s sake, we should hope some version of that trend returns.
It is not black and white, though (excuse the pun).
Imagine for example a piece of software with a settings menu which maybe gets used once per user, if ever, and it is only for technical stuff (think: assign more RAM and change V-Sync method). For that use case, a simple, subtle icon which is perfectly findable for the target group in the rare case it is needed is perfect. A colorful, detailed icon might disproportionally distract and thus diminish user experience.
Point is - use the right tool for the right job, and thanks to the author for providing a specific tool!
