Sorry, I misunderstood. Yes, of course you’re right that if you are using cellular for the modem and the modem to reach the Internet, that definitely does help.
The ADT/SmartThings system is using their cellular to call the monitoring center directly.
Any other smartthings hub model uses the cloud to initiate contact, Including to Noonlight.
As far as how often the smartthings cloud is unavailable, that’s documented both on the official status page (although that doesn’t catch everything) and on the first bug reports page in the community – created wiki.
For the last 20 months it has been down at least once a month, sometimes briefly, sometimes for more than a day. If you are not at home and the outage is brief, you may not even realize it went out. But that’s less of an issue for Home Automation than it is for security.
In addition, there has been at least one and typically more than one scheduled outage every month over the same time. Every time they announce a hub firmware update, that will mean The functional equivalent of a cloud outage.
So it does happen, and it happens pretty often. But it may be short enough that you don’t notice. Whether that matters to you in regards to a security system or not, again is just something that varies by individual.