To be honest, I don’t personally use smartthings as a security system at all. But for people looking for a low-cost DIY option with a professional monitoring center, I do suggest the smartthings/ADT model as a potential candidate, along with Ring, Abode, and Simplisafe. (I don’t include nest because professional smoke detector monitoring is very important to me, and while nest has excellent smoke detectors, they won’t trigger alerts to the professional monitoring system from them because they don’t have a cellular option.)
As for a choice of zwave and zigbee home automation devices with the classic app, it’s huge because of the custom code capability. Individual community members have created their own device type handlers for all kinds of interesting devices that aren’t officially supported, and most share them freely with other community members. You can see some of the lists in the community – created wiki:
There are other competitor hubs that support even more devices just because they support more different protocols. Smartthings has a Bluetooth radio, but you can’t use it for anything. Lutron integration is cloud to cloud through the lutron SmartBridge. There’s no IR bridge other than the Logitech Harmony, and no 433 MHz options.
But when it comes to certified Zwave devices and certified zigbee home automation profile devices, The ability to use your own custom code really expands the selections available. And the community is very creative and helpful.
But again, Samsung also tells us in this forum that their typical smartthings customer has 15 or fewer devices and uses no custom code at all. So they have not chosen to carry all of those capabilities into the new app or the new cloud platform.
I know that doesn’t really help with the decision-making process that you’re going through, so I’m not really sure what to say to be helpful.