I don't understand the difference between 'sudo service nginx restart' and 'sudo systemctl restart nginx' and also 'sudo killall nginx' still works with systemd. So I'm not understanding the simple use example.

Whether SysVinit scripts or systemd scripts, you have to learn something to do either one. I just see it as a difference rather than which is simpler. But systemd can use SysVinit scripts anyway if you prefer it.