Smartthings has multiple specific reliability issues. For one thing, you can neither defer nor deny updates, which historically have happened at least once a month and then will take your hub off-line. Maybe it’s only for a minute or so, but it’s often longer, and of course if there’s a problem it can be much worse than that. The problems don’t happen to everybody every time, but they do happen to some people. 
Smartthings is also primarily a cloud-based system, which adds additional points of vulnerability.
And historically smartthings has from time to time made backend changes without documenting or announcing them which then caused some loss of functionality for some or even all customers. 
So advanced users choose the smartthings platform for its versatility, not its reliability.
There are definitely alternative platforms which are much more reliable, but they may have less versatility and support less rules complexity. So it just comes down to your own needs and preferences.