A lot of those smart thermostats do not support things like OpenTherm. As a result they often either start the heating or they do not heat. There’s no modulation.

OpenTherm is from 1990s. Having a thermostat with a presence sensor is also not really anything new. Adding remote functionality would be nice if those “smart” wouldn’t be so utterly terrible at being a thermostat. Meaning, the lack of modulation. And no modulation is pricy way to heat your home.