Was thinking of this the other day when my hub went offline (red light) overnight the other day after the latest f/w update.
With the global mail system currently under stress, I was wondering that it might be a good idea to get another hub as backup in case mine dies. I have backup devices for those that run 24/7 like my network gear.
With more of my house slowly becoming dependant on HE it would be a disaster if it failed!
For a device you anticipate running for years maybe it would be a good idea to have a spare on hand. You might want to activate it though just in case. Hopefully they will have a way to migrate things before you need it.
I have 2 C4s that are my "production" hubs so I can theoretically switch over to a C5 with an OTG cable and "restore" from an external backup if needed.
edit: I have 3 C5's - 1 as "a control" hub (not using radios) which is seeing a lot less use these days and 2 spares.
I have not tested this but others have. The C5's can still use the Nortek stick attached via an OTG cable - the European C5 hubs also have external sticks with internal radios disabled I believe. Since all the data is on the stick a restore simply puts the rest of the config on the hub per usual.
The staff at HE seem to have mixed feelings about this though due to extra failure points and the possibility of a future migration tool (nothing confirmed sadly).