At the first glance, it is a welcome feature to let the SMB update itself to the most recent firmware. Of course, there is always a chance of side effects ;-) Usually, the update and reboot can occur at a scheduled off hour, so the impact is very low as the reboot takes about 3mins.
In a HA config with clustered SMBs we could expect the update to be handled differently on actve and standby node. But firmware is not synchronized, each member has to upgrade individually. It is not possible to set a different time for the search for firmware updates on the nodes, so both will always have the same time for update and make that three minute gap inevitable. A possible workaround is to set the date on each member with ~5m difference.
This a a CP statement from Mai 2015 - i just wonder if it is still working the same way, but i see no new feature that could help here.
An alternative is not to schedule automatic firmware updates in WebGUI, but to trigger it by scripting - Embedded GAiA lets you activate immediate search and install of firmware updates usind CLI:
set cloud-services-firmware-upgrade activate true frequency immediately-when-available
After a certain number of minutes, you can switch it off again:
set cloud-services-firmware-upgrade activate false
This can either be perfomed using any GAiA devices System Management > Job Scheduler page or by activating the crond on the SMB unit, see my CheckMates article Perform scheduled scripted tasks on SMB devices.