I have followed the development of SerenityOS and think it is a very interesting project. In short it is a unix-like OS but unlike *BSD and linux, it is intended to be a complete OS where the kernel and userland (all the way to the GUI) is very aware of each other, not unlike MacOS. The main developer, Andreas Kling, has worked for Apple and really has a full vision on how everything should work.

So in context to the topic, this approach, optimizing the kernel for userland and vice verse, the complete system could be very efficient, streamlined, stable and consistent, but i guess the drawback will result in a monolithic project where it will be difficult to add independent userland OS-components (like init systems, display servers, and maybe even UI toolkits) that are better then built in ones. But as far as i understand this is exactly what Andreas wants, and what he consider being the biggest advantage MacOS has over other OS. Also the GUI is inspired by Windows2k.

SerenityOS is just about a year old now, but they have already been able to install it on actual hardware and gotten doom and quake to run. They are currently building a browser. Andreas uploads daily videos to his youtube channel that i cannot recommend enough.

https://www.youtube.com/watch?v=gZ2gBVzIjBU