In default configuration php-fpm produces errors 502(bad gateway) on reloading. This is really annoying, if you need to do it often. For example, if you use tool like deployer to deploy your application into new directory, switch symlink to it and reload php-fpm.
Problem is in php-fpm parameter
process_control_timeout(documentation). It controls time for child process to process signals from master and defaults to 0. So effectively reload kills worker, and you get errors. I recommend setting this parameter to same value as
max_execution_time, so worker have time to finish processing request.
One more nuance about php-fpm: unlike other Unix services, it is reloaded on SIGUSR2, not SIGHUP. In Ubuntu, old upstart versions could not have custom signals for actions. In later version in was fixed, but service description file
/etc/init/php7.0-fpm.conf is still broken. For Ubuntu 12.04 and 14.04 you should add line
reload signal USR2 to the file, otherwise
reload php5-fpm will effectively restart it. In later Ubuntu versions systemd is used, so this problem is irrelevant.
UPD: TomCanBe in reddit comments mentioned, that in most cases php-fpm reload is not necessary, because resetting opcache is enough. This can be done with
opcache_reset() PHP function(should be called from web, not from cli) or with special tool cachetool.