It just comes down to what you need for your peace of mind.
Personally, my requirements for reliability for home security system are much higher than what SmartThings Provides at this time.
For example, I have my SmartThings SHM set up to send to notifications, one when the gate is left open, and one when a particular closet is left open.
The one for the gate works fine. The one for the closet door doesn’t work at all. I have been working with support on this, they were able to duplicate the problem, but they don’t have a fix.
So I’m OK, I know the notification doesn’t work, it’s more of a reminder anyway. But I certainly wouldn’t be using it for a personal safety situation.
In general, systems for true home security have a very limited set of devices So that they can do the same levels of reliability as a smoke sensor. They are built for that purpose and only that purpose. They do not rely on the Internet to send notifications. Some competitors in this area would be frontpoint, simplisafe, and scout alarm.
Then there are home monitoring systems which don’t reach the same level of reliability, just give you some peace of mind. Canary, for example. Personally, I would put SmartThings into the second category. It will give you some good information much of the time. At quite a low cost with no monthly contract. Many people find that valuable.
Whether that’s enough to meet your own needs is something only you can decide.