Your system has had some of the problems, you just might not have been home at the time. There have been four or five major issues that affected every customer, but, again, if you weren’t home you wouldn’t have seen them.
And of course there are other problems that might affect parts of the system that you are not using. Daylight savings time issues hit everyone for several years in a row. This year it only hit some people. But if you didn’t happen to have any rules set up that depended on those, or, again, if you weren’t home at the time, you might not have noticed. 
Smartthings has had at least one outage every month for the last 21 months except for January 2017. These are documented both on the official status page and on the wiki’s “first bug reports“ page.
I don’t doubt at all that your perceived experience has been that everything has run smoothly. Smartthings is a very powerful, highly configurable system, or at least it was until the new app, so perceived experience can definitely vary quite a bit. 
