Well, for now it is that easy, but for most people, the automatic update process works fine on its own.
There had been issues where Macs which were pointed to a private software update server, as is common in business and education, would not get the Xprotect and gatekeeper updates automatically. I've instituted a different system to get these updates installed on the systems I manage so I can't say for sure whether that's been resolved as of now.
The challenge with invoking the updates through softwareupdate manually (as well as downloading and installing the packages manually) is that these are undocumented and unsupported means of getting important security updates. Apple has a tendency to break these sorts of things.