SELinux also has been integrated into major distributions (Fedora/RedHat/...)
in much early phase.
So, in past, it made various kind of troubles, so SELinux developers have
continued to improve it for a long time.
Nowadays, we can see few cases which need to write our custom security
because the default security policy provides various kind of predefined
security policies. We can choose them as necessary, without writing our
custom security policy module.