Different things work for different people. And different people have different priorities.
The Schlage is a nice lock, many people use it. 
The SmartThings system is not UL listed as a security system because of some obvious deficiencies. The battery backup only works for about four hours ( the UL standard requires a minimum of 12), and it has no means of notification when it is on battery back up if its cloud connection is down. It will not send you texts, phone calls or push notifications. instead, these will all stack up, and will all be sent at once when your Internet connection is restored.
This is very different from most UL listed security systems like simplisafe, scout, and frontpoint which have cellular connections that are battery-operated and so can continue to send notifications even when the power is out.
While it is certainly technically possible for a system to maintain communications between its phone app and the hub even if the Internet is down as long as your local Wi-Fi is still operating, SmartThings does not do this. Instead, if the SmartThings cloud is unavailable, your phone will not be able to talk to your hub until it is restored.
There are a few things that can run locally in smartthings, but not many yet. They have said that they hope to add more in the future. Right now there are some lighting automations that run locally. I am not sure if there are any sirens that run locally or not.
Another thing to know about SmartThings is that they do push out updates without advance warning, and you cannot delay them. So your system may go off-line at any time for 15 or 20 minutes. Sometimes even for a couple of hours. If you live in a truly bad neighborhood, that’s something to be aware of since most security systems do not shut themselves off in the middle of the night. But SmartThings may.
There are certainly still people using SmartThings for home monitoring who are happy with it because it is so cheap. Again, different things work for different people. It is important to understand what you’re getting, however, and the difference between this and a UL listed security system.
As far as specific devices, the first place to look is the official compatibility list. You will find many excellent battery operated contact sensors and motion sensors on the list, as well as a couple of sirens. With SmartThings, you can choose either zwave or Zigbee.
At present, there is no keypad on the official compatibility list. Some community members have been able to make the iris keypad work with smart things, so that might be an option. You can search the forums for more information about that one.
https://www.smartthings.com/compatible-products/
IFTTT is not typically managed at the individual device level, but rather at the hub level. SmartThings does have an IFTTT channel, which can certainly be useful. IFTTT itself, however, always requires an active Internet connection, so didn’t really seem to fit the rest of your use case.