Android (operating system)

the best full featured free OS we have today, since POSIX gave up short of any UI specification, and Chrome OS is not there yet
usable and likely efficient Java API for apps if Oracle doesn't manage to destroy it with its lawsuit

However, many, many, many terrible horrors come with it:

it hasn't made the move to desktop for too many years. It could destroy Microsoft Windows and replace it with open source, but they just won't budge towards an unified mobile/desktop setup.
vendors litter it with uninstallable bloatware that should be illegal. European Union to the rescue!!! www.cnbc.com/2020/12/15/digital-markets-act-eus-new-rules-on-big-tech.html
vendors lock down devices so it is very hard to get sudo, let alone to modify their images!
there isn't enough hardware standardization for open source distros to thrive like on desktop
code drops mean that "master" is useless and trying to contribute from outside vendors' closed walls is a waste of time: stackoverflow.com/questions/1809774/how-to-compile-the-android-aosp-kernel-and-test-it-with-the-android-emulator/48310014#48310014
if you ever go below the Java API, e.g. to C++ or AOSP build, everything is horrendous and undocumented
Google doesn't care about the CLI, even the hello world requires creating infinite out-of-control boilerplate from a GUI: stackoverflow.com/questions/20801042/how-to-create-android-project-with-gradle-from-command-line/46994747#46994747
the boot is uber bloated and takes forever in cycle simulators