The smartthings models other than the ADT are OK for convenience self monitoring but can’t be considered a security system. They just lack basic features When compared to purpose built security systems like ring alert, simplisafe, Abode, etc.
Much discussion of this in the forums already, but it comes down to some very basic features which are just missing because of the cloud dependency. Again, see the comparison FAQ:
For example, if either the Internet or the SmartThings cloud is unavailable there is no way, none, to get any of the notifications that a self monitoring system would depend on. Because in SmartThings all notifications come from your cloud account.
There is also no way to arm/disarm The system locally. Again, none. Multiple postings in the forum about people who came home, the Internet was out, and the only way to get their individual sirens to turn off was to go around and take each one off power individually. 
The classic app doesn’t offer a locally operating entry delay. The new app does, but doesn’t offer arm/disarm based on Geopresence.
I personally really like smartthings for convenience notifications, like getting a notification if the guestroom window is left open, rain is expected, and the guest is away from the house.
But for security, the only model I think can be a legitimate security candidate is the ADT model hub.
Plenty of people use SmartThings for convenience self monitoring, and there’s no problem with that as long as you understand that what you are getting does not meet the minimum definition of a security system for most people or for the UL standards. 