I just tried this way to update on my RT-AC68U with firmware version 384.11_2.
The firmware version 384.12.0 was downloaded automatically and "installed" without any additional steps from my side during couple of minutes.
Then the router rebooted as usual but firmware version didn't change. At the same time my USB disks weren't mounted.
So I tried to repeat these actions one more time. But without success (this time my USB disks were mounted correctly).
Then I tried the third time
But the result didn't change.
It seems as this way to update has some errors which influence on the final result. But it didn't harm my device anyway and there is a sense to try on your devices
p.s. During update over web interface I faced the same problems. They caused by JFFS custom scripts, so I formated this partition, rebooted device and repeated update over web. Result was successful - new firmware installed. After that I restored my JFFS custom scripts and rebooted device.