My Ecobee thermostat -- which is reasonably usable without an Internet connection -- has one horrific flaw: the built in clock seems to drift by a minute per month, leading to my programmed schedules shifting ever so slightly.
I could have it connected to a dedicated IoT SSID and live in a VLAN jail so that it only has access to my NTP server.... or I just change the time manually every six months as part of DST.