Under Upstart, finding out why a service failed to start meant knowing which log file it wrote to. systemctl status prints the exit code and the last ten journal lines for that unit, which is usually the whole answer.
$ systemctl status php7.0-fpm
● php7.0-fpm.service - The PHP 7.0 FastCGI Process Manager
Active: failed (Result: exit-code) since Tue 2016-09-13 09:14:02 UTC
Process: 2841 ExecStart=/usr/sbin/php-fpm7.0 (code=exited, status=78)
Sep 13 09:14:02 web php-fpm7.0[2841]: ERROR: [pool www] cannot get uid for user 'deploy'The last line is the failure, printed without anyone having to guess at a path. -n 50 widens the window and -l stops it truncating long lines, which it does by default and which hides exactly the part you need. For anything older than the last boot, journalctl -u is the same information without the limit.