For some people, the range of configuration options, and the behavior of smart energy saving features, can be a bit much. A properly configured device, though, can provide flexibility and save considerable energy wastage by avoiding running the system when no one is home, or when whether conditions may warrant otherwise.
If you just want to set a temperature and have it stay there, not matter what, then a smart thermostat probably isn’t for you, and the added cost overhead would be better spent elsewhere.