SmartThings itself does not provide the kind of uptime most people expect from a security system. The fact that they push out updates that take your system offline for anywhere from 25 minutes to a couple of hours, often with no warning, takes them out of the security category for me. (Just this week there were no notifications for a couple of hours one afternoon.)
But if you are going to use it (different people have different requirements), most people who don’t use SHM use SmartAlarm… No reason to reinvent the wheel, when a great one already exists.