Lots of good questions! 
I’ve moved this to projects since many of the answers will be a matter of opinion, and different people will have different things that they like or dislike. Also, that way you can get individualized responses based on your own preferences.
To take the easy questions first, as far as delay in zwave or zigbee signals being passed around the mesh, there can be a slight delay, typically maybe two or three seconds. Shouldn’t be longer than that.
With regard to coverage area, see the FAQ:
As far as motion sensors, the short answer is that there are no really good Z wave motion sensors for use outdoors. But there are other options you can use. Again, see this FAQ.
Now to some of the more complicated questions. The fact is, SmartThings simply isn’t really suitable as a primary security system. To begin with, it’s cloud-based. If both the SmartThings cloud and the Internet are not available, you can’t get any notifications at all, not even on your own phone even if it’s on your own Wi-Fi. The battery back up is only good for a few hours, maybe 3 or 4, but it can’t send notifications as there is no cellular option at all. Not only that, when the system is disconnected from the SmartThings cloud there’s no way to either arm or disarm the security system. The mobile app on your phone cannot talk to the hub at all without the cloud.
On top of the basic design issues, the SmartThings system itself has some reliability issues. There has been a major outage every month for the last eight months except for January. Sometimes it only lasts a few hours, sometimes it’s a day or so, but it’s not what you want from a security system. In addition, they can and do push out firmware updates, typically one or two a month, which will take your system off-line for anywhere from 15 minutes to a few hours and you can either refuse nor postpone these outages.
So most people who really need a security system choose something else. They may then integrate it to some degree with their SmartThings system, just for convenience sake.
The SmartThings system is not UL listed as a security system, and with its current features and limitations it could not be. There are a number of other systems to choose from.
All of that said, I do like SmartThings for complex convenience notifications, such as notifying me if the guestroom window has been left open, the guest is away from the house, and rain is expected. It does this kind of thing much better than most systems in the same price range. But it’s not a disaster if it fails to notify us.
If you look at the SmartThings product usage guidelines, you’ll see that they say in two different places that the system should not be relied upon for security uses.
https://www.smartthings.com/guidelines
Data accuracy and consistency from SmartThings sensors, including those provided by SmartThings directly, resold by SmartThings, or supported by SmartThings, is not guaranteed. Therefore, you should not rely on that data for any use that impacts health, safety, security, property or financial interests.
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For example, because temperature readings may vary significantly from reading to reading on an individual device, between devices, or
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Alternately, presence data from SmartThings devices or mobile/Smartphones can vary in accuracy, and therefore should not be used to control access to secure locations without secondary authentication.
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SmartThings provides information on the physical state of many devices, including through contact sensors (which may provide open/closed status of windows and doors), accelerometers, locks, appliances, HVAC, light and power fixtures, and presence sensors. The complexity of combining physical interactions with digital interactions may result in inconsistencies in the representation of the actual state of a device in the physical world in its digital representation.
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In all cases, SmartThings does its best to interpret and maintain the state of these devices (including by querying for current data), but this is not intended as a replacement for direct, physical verification in situations where the true state of a device may have an impact on health, safety, security, property or financial interests. For example, you should not assume that a curling iron (which may cause a fire if left on too long) is actually off without physically verifying the state.
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While SmartThings supports multiple communications standards, including ZigBee and Z-Wave, we cannot guarantee the implementations of those standards by third party devices. Certain devices may not work, or may cease to work with SmartThings despite supporting the same standards.
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We provide no guarantee or warranty of compatibility for third party devices, even if we provide access to or resale of those devices through the SmartThings Mobile application or ThingStore.
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SmartThings allows for the control of electronic devices through pluggable outlets and supported third party switches, outlets, relays and controls. Where possible, SmartThings has worked to ensure message delivery and control reliability, but many factors can impact this, including electromagnetic interference, solar flares, wireless and cellular connectivity, and the natural complexity of software instructions that might be in direct conflict caused by SmartApps or by end users.
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These and other issues can cause commands to not be delivered, to be delivered repeatedly, or to be delivered in rapid succession. Therefore, do not use SmartThings to control any device that may have an impact on health, safety, security, property, or financial interests, or where the device is sensitive and could be damaged by power cycling or power surges. For example, do not control medical equipment or expensive electronics with SmartThings.
So it’s a useful system for many things, but not really recommended for primary security, even by the company itself.