I’ve an extreme old-timer on HA, having been around for the early BSR X10 days. I’ve automated, to various degrees, four successive homes of mine, with technologies moving from that X10 (CM11A controller initially, up through Adicon Ocelot/Leopard), to using fledgeling Insteon and HomeSeer, with forays into UPB, ZWave and even hard-wiring. I wrote a popular plug-in, PowerTrigger, that brought flexibility and also security camera integration into HomeSeer back in the day. And I wrote LANNouncer for SmartThings plus Android.
In not-so-short, this ain’t my first rodeo.
If you’re really looking at security, you can’t use SmartThings. You need better reliability.
If you’re really looking at full home automation, for a full home, you can’t use SmartThings. You need more control.
SmartThings is entry-level home automation. Mostly easy to use, you don’t have to do any programming to use it adequately, but it doesn’t get you very far. Even with programming, it doesn’t get you very far in terms of ease. Although the IFTTT and Alexa integration can provide a simulation of ease.
HomeSeer, in contrast, takes much longer to set up and a basic setup for SmartThings-equivalent functionality will cost you about six times more than SmartThings. So many things are a $50 add-on; that’s half the cost of SmartThings right there. And the hardware - you either need a PC running it 24/7 or to buy one of their hardware controllers, which are also basically PCs - so that’s a cost. But… you aren’t reliant on their cloud. It works, and it works quickly. You have far more power. And it will take ten times longer to get it doing the first bits.
This isn’t really a ding against SmartThings. SmartThings is great for adding automation to an apartment. But if you want to go much further, you’ll hit limits pretty quickly.