Yeah if you do your scheduling with ST, you’re beholden to the ST scheduler. We know that ST is working on improving the scheduler, but no one really knows how long that will take to get to a truly stable place. Plus, if your internet fails, you lose all scheduling and control.
I prefer having my HVAC on a separate, purpose-built system. I live in Chicago, so if my thermostat fails when I’m out of town for Christmas, we’re looking at burst pipes. With a c-wire, my Nests have been solid. I don’t trust ST quite enough yet. Plus with Nest, my utility pays me money every summer during high demand.
There may be z-wave thermostats with programmable safety temperatures (i.e. always run the heat no matter what if the temperature in the house is below 50°), but you’d have to ask others.