I am contemplating dropping the Iris setup in favor of ST’s as both an alarm and smarthome but have a couple questions. Can ST’s be set to recognize several phones on the system so it can arm/disarm itself with no input from the operator? From reading this forum it doesn’t sound like there is a good keypad available yet, true? Will the ST hub work as an alarm when offline, no internet? (i.e sound an alarm should a contact sensor open). That’s my start anyways…
Yes I have 3 phones on my ST setup for presence. And the Iris 2nd gen/centralite keypad works well with ST. The SHM which is ST built in alarm has had lots of issues. Smart aLarm is an app and works well. Neither work offline as far as i know.
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(I’ve moved this to projects just because we’re covering a lot of different aspects here, some having to do with smart apps, some having to do with devices, some having to do with the platform itself, so it’s going to be easier to get you all your answers in one place in this category. )
Last things first: the iris keypad can work with SmartThings, it just needs a custom device handler which has already been created by a community member.
One of the biggest differences between SmartThings and Iris is that customers have the ability to upload both custom code and custom device handlers to their account. This means that you can use devices that are not on the official list, particularly if they use one of the established third-party standards that smart thing supports like zwave and Zigbee home automation (ZHA1.2).
Many community members enjoy the challenge of creating new device handlers, and will upload their code for other people to use.
The following thread gives you an overview of how custom code works in smart things.
If after reading that it looks like something you’d be interested in, there is a section in the forum for community-created device handlers:
https://community.smartthings.com/c/projects-stories/community-created-device-types
And here’s the specific one for the iris keypad. 
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The short answer is no. SmartThings offers very little local processing, does not have cellular communications, and although with V two there is a battery back up, it’s only intended to last for a couple of hours and, again, most things can’t run locally. In particular, SmartThings has no way of sending notifications without the Internet.
The longer answer depends on the exact details of what you want to accomplish
The longer answer is maybe, sort of, if you add additional equipment. (Although the V2 hub has two USB ports on the back, they are not enabled, so you can’t use those for anything.)
Some people have connected SmartThings to some degree with other security systems such as Simplisafe or the official scout integration (for which you pay an additional monthly fee) but the problem is these all rely on the Internet. If the Internet is down you can’t communicate with the other system.
I know SmartThings really wants to sell itself as a security system, and since there is no official definition in most states they can do that, but you need to check carefully to see if the features that you want are provided. Everyone has different requirements for their own peace of mind.
For myself, I am quadriparetic, and have housemates who have very irregular schedules , so professionally-monitored fire safety is very important to me, and needs to work even if the power and Internet are down. I pay a separate fee to a separate security system in order to get that. Like most people I feel that I’m paying a little too much for the value that I receive, but on the other hand, I have the features that I want, the system is been extremely reliable, and I’m happy with everything except the price.
Smartthings is my favorite home automation system when it works as designed. They have had a number of reliability issues, which they are very aware of, and the company has committed to improvements in that area. It’s still early days for that, but I definitely hope that they succeed. But as a practical matter since last November I have yet to go 10 days without an impactful SmartThings failure. So I do use it for non-critical things like knowing if the window in the guestroom was left open when it’s going to rain. But I don’t personally put my security system on it. But that’s just me. Other people have other priorities and other feature requirements.
The other thing to be aware of, particularly if you live in a high crime area or you have a personal reason to believe you might be targeted, is that smartthings will push out platform changes and even hub firmware updates, often in the middle of the night, and often without prior notification. That means your security system might be down altogether for anywhere from 10 minutes to several hours. While you’re asleep. 
Many people, including myself, have requested a way to postpone these maintenance updates, but it doesn’t look like that’s on the roadmap. So again, just something to be aware of. It won’t be a problem for everyone, but it will be an important issue for some.
And the medium answer
If you select your devices very carefully, and you really mean only have a siren go off for the contact sensor opens, it is possible that you could set things up so that when a sensor reported activity a siren went off, even if you had unplugged your smart things hub altogether. ( you still won’t get notifications, but you can have one device automatically trigger another nearby device by using the standard protocol features. Assuming both devices are compatible and all that.)
The problem with this approach is that it removes all of the conditional logic. Every single time that sensor reports activity, that siren will go off, no matter who is home, no matter what the time of day, no matter any other conditional features you might think of. That’s because with this approach you are bypassing smart things altogether.
It can still be useful for things like security on a garden shed, but it doesn’t meet most people’s requirements for residential home security. But I just mention it because it is possible. So you have to be a little bit careful about what you ask for.
Can you set it up so a siren will go off when a specific contact sensor opens? Absolutely.
Can you put all kinds of conditions on that event? Absolutely – – but only if you are letting the smartthings hub handle the logic which probably means your Internet and power are up. If you want that event to happen when the Internet is down, you might be able to do it with some conditions using the limited local processing available to the hub. But it’s a little more likely that you would set it up with either zwave association or Zigbee binding, in which case you lose the ability to “disarm” it under specific conditions.
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Awesome, well thought out and helpful response. Thank you for taking your time, it told me just what I needed to make an informed decision.
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