Is there a Smartthings official view on what we should expect if cloud connectivity is down for a period (seconds, minutes, hours)
I am hoping for an answer that specifies effect on routines/automations etc
Things will be different after the transition to the new architecture is complete, so I assume you were asking about events at that time.
Under the new architecture, most routines created through the smartthings app will run locally, and those will continue to run provided all of the devices involved are also running locally. Of course if a device requires a cloud connection and the Internet is out, the routine will not be able to complete.
The SmartThings app, however, always requires a connection to the smartthings cloud, even if it is on the same Wi-Fi as the hub. Technically they didn’t have to design it that way, but they did.
Here’s the official schematic for the new platform. See the app in the upper right?
You will also note that all custom smartapps require a cloud connection at the present time. That’s because that is where the smartthings API processing is hosted. A few community members have been doing some very clever things with edge drivers (which are written in LUA and run on the smartthings hub) and servers they have set up on a local LAN connection, and this can create some local processing in specific instances. This includes an MQTT bridge. But it can get a little complex. To find these, use the quick browse lists in the community-created wiki and look under miscellaneous.
How to Quick Browse the Community-Created SmartApps Forum Section - Things That Are Smart Wiki
Without a connection to the smartthings cloud, you will not be able to add new devices, run new routines, or do anything else which requires the app.
It is not yet clear whether you will be able to change the security mode for the smartthings home monitor. You could not under the old architecture, that was a cloud operation. But we don’t know yet how STHM will operate under the new architecture.
So, long story short, previously created routines should run as long as the devices they use are eligible to run locally and they don’t have any special features which require the cloud (which have not yet been fully documented). It doesn’t matter whether the Internet is unavailable for a second or minute or days, it’s all the same answer. Nothing that requires the app will be able to run, and most custom automations will not run unless specifically designed for that use case using a separate local server.
But we just don’t have all the details of everything yet. 
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Many thanks. Very helpful
Just in case anyone wants to update the diagram it doesn’t flag the hub connected routines/automations/rules that will operate locally
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