Check out the video for a more in-depth description of the problem, but the short of it is that most smart thermostats (and a lot of smarthome devices) rely on someone else’s servers in order for them to be accessed remotely. And because of this, “remotely” doesn’t just mean when you’re out of the house, but inside the house as well. If the “smart” device company ever goes out of business or decides to stop supporting whatever you own, then you effectively will no longer have an internet-enabled thing.
This isn’t true for all devices. Some do not require servers-that-are-elsewhere (or “the cloud” as it’s known) so that they can operate. But a surprising amount do, and that’s something to consider when buying an appliance, thermostat, Echo, or full home automation system. Will the company running those servers still be around in 5 years? In 10 or 20 years? And even if they’re in business, will they support it? With something like the Amazon Echo, that’s not much of a concern. But with a $250 thermostat from a “new” company it could be a factor.