I feel your pain. This isn't intended as a flippant answer...it's the SmartThings architecture that is responsible, and the smartthings team that is working very hard to improve the stability and reliability of the system. But they're not there yet.
Just this month, two things happened. The SmartThings CEO posted in these forums to apologize for the instability and to assure everyone that they are making reliability a top team priority. And as part of demonstrating that, they hired a new chief of engineering who was the chief of engineering at Amazon for their technical initiatives, including echo. All of this to help them scale their solution. So they do realize that they have problems, but they don't yet have a solution.
A lot of the instability comes in a "musical chairs" format where only some customers get hit. (If you're interested in technical details, google "Cassandra hotspots".) sometimes one region is more affected than another, but often it's just completely random. Which adds to the frustration because when you post about a problem in the forums, probably the first few answers you get will be people saying everything is just fine with their system. Which it may very well be. Even if they have a set up that is identical to yours and they live in the next house over. There's just a high degree of randomness to what fails when.
For now, the best thing you can do is to send in a support ticket to email@example.com or firstname.lastname@example.org depending on which region your account is in.
You can also check the status page at http://status.smartthings.com , just be aware that the company has a philosophy that only problems that are affecting almost all customers get posted to that page. So even when it's a known error of the "musical chairs" variety, it won't get posted there.
When SmartThings is working as intended, it is absolutely my favorite home automation system. It is flexible, versatile, the team has a great vision, the community here is excellent. What it isn't yet is reliable. Since last November I have yet to go 10 days without an impactful SmartThings failure. And I have a very simple set up with almost no custom code.
I do believe them when they say they intend to make it much more stable, and I look forward to that day. In the meantime, it's up to each person whether the value they receive is worth the amount of time and effort they have to put into maintenance and the amount of patience they have to muster to get through the unresolvable glitches until the system rights itself again.