SmartThings is not a substitute for Simplisafe (or any other UL listed home security system).
Smartthings lacks several things that a UL listed security system would require. The most important thing to understand is that it is still primarily a cloud-based system. If the Internet goes out, even if the power is still on, you will not receive ANY notifications. You will not be able to change the alarm mode to arm or disarm the system. You will not be able to use any keypad unless the Internet is available and the SmartThings cloud is available.
Also, the current system does not have a battery back up.
There are other issues as well. So while there are some people who use smartthings for notifications, it’s not usually as a substitute for home security. It’s just not built for that. That’s not just my opinion: the company says so themselves in their product usage guidelines
Data accuracy and consistency from SmartThings sensors, including those provided by SmartThings directly, resold by SmartThings, or supported by SmartThings, is not guaranteed. Therefore, you should not rely on that data for any use that impacts health, safety, security, property or financial interests.
( The exception to all of this was the dual logo ADT/smartthings system which is still listed but is essentially “end of life“ and is now very hard to find for sale.)
Anyway, if you are looking for an inexpensive DIY alternative to simplisafe, I would suggest either the ring security system (typically the highest rated inexpensive system these days, including at the Wall St. Journal and consumer reports) Or the Nest system. Neither integrates directly with smartthings, but both are much better security systems.