I can’t comment yet on the long-term reliability of it (I only started on this project in January this year) but I can say that I’ve been using it at my house with 100% reliability for a couple months since I worked out all the kinks. Yes, I’ve added some software features to solve the stuck in the wrong state problem.
My personal opinion (and I think I may be in the minority here) is no. In fact, I was in the same situation as you when I set out to create the NodeMCU-Smarthings project, my house had a built-in alarm that we never used. I think the usability of those DSC panel systems sucks. Who wants to type in a code every time they come or go? I put it to the wife-and-kids test … is my wife going to remember or care about arming the DSC alarm system every time she leaves the house with two kids to strap into car-seats? In my case, definitely not.
The best thing about a SmartThings-only alarm system is the full automation of it. This was my goal when setting out to build this project. Once it’s set up, you never have to do anything to arm or disarm the system. The wife and I both have the ST app on our phones set up as presence sensors, so the system arms and disarms automatically when we come and go.
Yes, the self-contained DSC system has an edge on reliability
Most of that big spool of wires coming out of your wall are going to the contact sensors throughout your house. It looks like your DSC system is pretty well labeled, those terminals labeled Z1, Z2, etc are the sensors for your zones and those are the wires you need to re-route to the NodeMCU. I can’t really tell from your pic which terminal controls the siren/alarm, but I’m sure you could figure it out.
Of course, one big advantage of the NodeMCU solution is the cost and simplicity. Assuming you already have a SmartThings hub and WiFi in your house, all you need is an $8 NodeMCU ESP8266 board (maybe a couple depending on the number of sensors you have), a $4 relay, and a few wires