Smartthings has had at least one outage in every month for the last 20 months or so except I think for January 2017. Not every customer is affected every time, and the amount of time varies, but it’s just nowhere near the standard of a pro security system.
Also smartthings can and does push out hub firmware updates which can either be denied nor delayed, and which will take your system off-line. Maybe only a couple of minutes, maybe a couple of hours. These have historically happened at least once a month. Again, just doesn’t fit with a security system.