syslog-ng service not starting with systemd but command works fine

We had the same problem on Debian 8.1, but fixed it by changing our syslog-ng local configuration to use unix-dgram instead of unix-socket.

I was clued in by this comment at RedHat Bugzilla:

Note about custom syslog-ng configurations files

People with custom syslog-ng configurations will most likely face upgrade problems due to the unix socket type mismatch between systemd and syslog-ng old configuration file:

• systemd creates /dev/log as unix-dgram
• syslog-ng < 3.2.5 expected /dev/log to be unix-stream (configuration file)

If you use 'unix-stream ("/dev/log")' in one of your log messages sources, you will need to manually change it to 'unix-dgram ("/dev/log")'.