Many users are using traditional alarm systems that have all the features such ass; Battery Backup, No Dependency on the “Cloud”, Cell Backup and Professional Monitoring to run in parallel with SHM. These traditional systems can not only be integrated in the ST environment but even the sensors can be used as devices in the ST platform for other HA purposes.
However, using ST, by itself, as an Alarm System, is not recommended. There are just too many things that can go wrong which may leave you, your family and your home unprotected.
I use a DSC Panel with 28 sensors and a device called Envisalink 4 through an always on Pi3 running Alarm Server to integrate my Alarm System into ST. Works great.
There are other solutions that work great as well. Some systems don’t even need a middle link as their is direct integration!
You have to search the forum, read up on options and decide which route you want to take.
I still pay my monthly monitoring fees but I am almost at the point where I am ready to drop it as I believe that I have enough security and alerts to monitor “Un-Professionally”…