I've just installed syslog-ng from epel onto a test CentOS7 box and found that the service wouldn't start, a quick verify of my syslog-ng.conf, with syslog-ng -s came back like so...

syslog-ng: Error setting capabilities, capability management disabled; error='Operation not permitted'

The debain fix suggests something like: syslog-ng -no-caps -s, which seems to work. What is odd is that /usr/lib/systemd/system/syslog-ng.service doesn't have it, but the service still seems to work once you've fixed and syntax errors!