Both the rpm/mongod.service and debian/mongod.service systemd service files start mongod with the --quiet command line option.

This is inappropriate because quiet mode is a user preference that, if desired, should be enabled in the config file. The older init scripts, including the init.d and upstart styles, start mongod with just the config file option (and have always done so). Thus the systemd service files should also start mongod with just the config file option.

Having --quiet enabled by default like this means that valuable information about connection lifetimes is absent from logfiles, which can impede diagnosis of issues.