Most smartthings models are simply not suited to being a primary security system because they are too cloud-based. You can’t even get a notification on your own phone even if you’re at home on the same Wi-Fi network unless the SmartThings cloud is operational. You can’t arm or disarm the security system if your Internet is out, a time when you might most want to.
If you follow any of the links about security on the SmartThings website now, they all take you to the ADT/SmartThings Model line, which looks like a decent competitor for a low-cost DIY system. It has a completely separate line of sensors which are definitely more reliable. It does have cellular communications to the monitoring center, but I don’t think that works for self notification on your phone, so it might not be a good fit for you.
https://www.smartthings.com/home-security
The people in the following thread are using that model and should know the details:
But I couldn’t recommend any of the other ST hub models as the basis of a primary security system.
Also, ST is in the middle of a major transition right now: new hub, new platform, new app. And the new app has many fewer features than the classic app did: for example, in the new app you are not allowed to arm/disarm on the basis of anything other than time of day. You can’t arm based on Geopresence when you leave the house, or with a different button, or anything else. You also can’t automate unlocking your smart locks. So I would definitely wait until all of those changes finish happening before investing a lot more cobbling together a security system based on it.
JMO