“UL Listed”
UL listing gets complex because it’s very specific about exactly what it applies to. When it comes to security systems, there’s one standard for the physical devices, another standard for the installation service, and a third standard for the monitoring center. So any or all of those might be UL listed. In addition there are different UL standards for single-family residences versus commercial buildings, etc. However, “hardwired” is not part of any of the standards. Both wireless and hardwired sensor systems can be UL listed. SmartThings itself is not UL listed in any of the categories.
Comfort Factors
When it comes to your original question, the answer is also complicated because it really just comes down to your particular needs and preferences as to whether or not you can feel “comfortable.”
I am quadriparetic (use a wheelchair and have limited hand function). So I use a lot of home automation, but I need it to be very reliable.
There are many things that can go wrong with SmartThings that might be very easily fixable by someone who was ablebodied. Maybe you just have to open the app, change a number, and save something again. Maybe you just have to take the batteries out of a sensor and put them back in in order to get it restarted. But while these glitches may be so easy to fix that many able-bodied people don’t even remember that there was a problem, I have to literally pay someone else to do that. So I am very aware of which systems have very few glitches and which have a lot.
When it works well, SmartThings is absolutely my favorite home automation system. It is versatile, it works with devices that I like to use, and it offers excellent trigger and notification options. However, it’s also the glitchiest system that I have. It was working really well in August last year, but then there were a number of major changes in the fall, and since October I have yet to go 10 days without having to call someone else in to reset something. And sometimes it just is unavailable for several hours or even days at a time.
In contrast, during the same period I was running Amazon echo, the Phillips hue bridge, some HomeKit devices, the GE Link Hub, Beecon+, the Kuna camera, staples connect, an automated sprinkler system, a home security system, IFTTT, Logitech Harmony, an Apple Watch, and a medical monitoring system, and none of them had more than one or two glitches in a period in which SmartThings had dozens. None of them have the same breadth of features that SmartThings does, or the same flexibility, but they were all more reliable. And not once did I ever have to have anyone else do anything to get those systems working again.
The glitches in the other systems were almost always when a cloud service was temporarily unavailable, typically only for a few minutes. With SmartThings, it’s quite common that when something stops working it doesn’t come back on its own. You have to do something to get it working again. What you have to do might be very minor for an able-bodied person, but it does take some effort, If only to track down what the solution would be.
(I should also say that my security system, for which I pay a monthly fee, has had no errors at all in about 10 years other than two false alarms. My biggest complaint about that is the same as everybody else’s– – I feel that I pay a little bit too much for the value that I receive. But I certainly can’t complain about the reliability.)
Luck is a factor
I think my SmartThings experience has been fairly common based on reports and the forums. There are certainly some people who’ve had almost no glitches, although they have still had some days of outages due to maintenance updates. And some people have had much worse problems where the system just couldn’t be restarted again without the assistance of support. But most people, I think, find that the system works quite well for several days or even a week and then weird things start happening (in the forums we tend to call them poltergeists) like lights coming on that shouldn’t or a particular set of devices no longer responding to controls. And then they have to try a few different things to get it going again.
And while I hope that stability will improve in the future, SmartThings senior staff have been promising an improvement in reliability for at least a year, and it hasn’t happened yet. Things get better for a few weeks, and then get worse again. So I just have to assess the system as it is now.
Answering two questions
So now you have to address two separate issues.
One) will the system meet your needs if it stops working two or three times a month until you go through A manual process to restart it again? For some people, the answer is sure, no problem. Given how inexpensive SmartThings is, and how versatile that is, they may still consider it a very good bargain.
On the other hand, if you have a child who is a bolter and you want sensors on your doors and windows that will notify you immediately if they’re opened, you probably will not be willing to risk downtime that can occur unexpectedly at any time given the recent frequency of those occurrences.
So different people will answer the same question about the same system differently. You have to think this through for yourself.
Two) are you physically able to do the manual resets that are required? Obviously this affects people like me, who literally can’t take the batteries out of the sensor. But it also affects people who have a location which is many miles away. It’s difficult to take the batteries out of the sensor to reset it if it’s a 4 Hour drive to where the sensor is located.
So you can see why different people might feel comfortable or uncomfortable with exactly the same system. Because it all comes down to their personal needs and preferences. SmartThings in its current state is definitely not a “set it and forget it” system. It’s not that hard to set up (it’s not that easy either, but people in the forums are always willing to help), but no matter how experienced you are, and how careful you are, it’s likely to require some babysitting every month. And it’s likely to fail a couple of times a month. Maybe not for very long, and maybe with an easy fix, but still it’s something to be aware of.
And sometimes the fix can take several weeks or even a couple of months just to get you back to where you were before. You can find lots of examples of this in the forums.
Again, for some people that will be just fine. They’re willing to do a complete reset of their devices every couple of months if that’s what it takes. And to reset individual things once or twice a month. Again, they appreciate the versatility that SmartThings offers and its very low price tag and still consider it very much worth the price. Other people will find the inconvenience outweighs the value and move onto a different system, potentially at either a higher cost or with fewer features.
Everyone has different requirements for peace of mind. With many different factors going into that. I can’t say what would or wouldn’t meet your standards for feeling comfortable but hopefully that will help explain why there are such varied answers. 
I hope it continues for you.