Very cool animal. I never had one of those in particular, but I had a number of different lizards and snakes when I was in high school.
My first thought is that SmartThings simply isn’t reliable enough to be certain of not killing the lizard by turning on the wrong thing at the wrong time. Or not turning something on at all. 
The company says the same thing, by the way, in their own product usage guidelines:
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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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. 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.
If you search the forums for “Reliability” you will find a number of reports of lights turning on unexpectedly, garage doors opening, etc. So it’s not just a theoretical concern.
I personally don’t put anything on SmartThings unless I would feel safe if it ran unattended for 24 hours. But that’s not true of heating lamps for a vivarium.
You are definitely right that you also have to be concerned about keeping the devices away from the animal, although there are a number of ways of doing it.
There are many different automated systems available for terrariums these days, and I think if you continue to do your research about the care of bearded lizards you will probably come across quite a few suggestions for ways to automate a temperature regulation system.
If you don’t look for anything too fancy, that is it doesn’t have to have a smart phone app or remote access, any good aquarium or reptile supply store should have a number of different devices you can get which just work on a mechanical timer and are very effective. I just wouldn’t add SmartThings into the mix. It’s too high a risk at this point.
REPTILES MAGAZINE is a good source of information for both care and equipment. Their beginners section discusses a number of options doing heating and specifically mentions bearded lizards.
