SmartThings is extremely versatile and flexible and it allows you to add a number of different kinds of devices. On the other hand, since November I have yet to go 10 days without an impactful error, and my experience is not atypical as you can see from reading forums.
Most security systems are extremely reliable, but get that reliability by limiting both what you can do with them and the specific devices that can be connected. It’s not uncommon for them to only offer a choice of two different kinds of contact sensors and one or two kinds of motion sensors and not much more. Even if they do decide to let you also do some home automation, they generally limit that home automation both in the number of different schedules you can set up and again, the types of equipment you can use.
So it really depends on what your needs are. Many people, myself included, use a separate security system and do not rely on SmartThings for that. But there are other people who do use it for security. Everyone has different requirements for peace of mind.
Have you looked at any of the popular DIY security systems like scout, simplisafe, abode, and piper? I think you would immediately see what I’m talking about in terms of having very few choices, but I also think that at least compared to the last six months, those systems would all be more reliable than SmartThings.
Personally, I have an additional requirement for security systems – – that they be able to operate when both the Internet and the power are off, and that they contact a professional monitoring center via cellular (not Internet). This would knockout Smartthings as a security candidate for me, because it has to have The Internet in order to send a notification. But again, there are other people who are fine with that limitation.
If someone is just intrigued by the idea of SmartThings as a home automation platform I usually recommend that they buy it from someplace with a 30 day guarantee, get two motion sensors, two contact sensors, an arrival sensor if you intend to use one, and a siren if you intend to use one (as long as you don’t have small children who might be frightened by the siren going off unexpectedly).
Then just use it for three weeks and see what you think.
on the other hand, if you really are looking just for a security system, I would at least take a look at some of the purposebuilt systems as well.