A Ninja file is probably no easier to write than a Makefile. But Ninja is a lot faster than make, which is the reason for using it. To see this, configure Chromium to build with Xcode, make, and ninja, and compare the build times.
Adding to this, from what I understand, ninja was designed to be generated by another more feature-full build tools. For example in chromium, its generated from gyp (generate your projects) files. The fact that its human readable is just a plus.
Makefiles I presume, will be hard to write by hand when:
1. Your project code base is large
2. It take large amount of time or very powerful machines to build sub-modules (V8 for e.g in chromium) so, you don't want to do it all the time
3. Conditional inclusion of sub-modules (for iOS vs Android if you have two separate set of files etc)
4. Sub-modules are pulled in from 3rd parties into your code base
I once wrote a custom build systems in python to generate Make files. I did this because I wanted to use convention instead of configuration and achieve fast builds on multicore machines. Later, after leaving that company, I wanted to recreate a similar build system. I wrote it in python to generate Ninja build files.
I found it easier and cleaner to write Ninja build files.
I wasn't able to do a direct comparison of the build speeds of the two systems, but I bet the Ninja builds were significantly faster.
