> installing a Linux distribution

Linux != Unix. Darwin is much closer to BSD.

> Also in El Capitan the root user is disabled by default.

Lies. I never touched a thing and (on 10.13 ß):

    $ sudo su -l
    password:
    Galileo:~ root# ps -u root | wc -l
    115

- You can't log into the UI as root (by default), and that is a good thing.

- SEP prevents you (or any random script/software/installer you run) from meddling with system files and that is a good thing too, no different from sensible SELinux or AppArmor policies.