Nest is designed to work with low-voltage heating systems (oil furnaces, whole-home heat pumps, natural gas, etc.) In contrast, Mysa is designed for high-voltage (120V-240V) heating systems that are not compatible with Nest. Systems like electric baseboard heating, in-wall fan-forced heaters, and some convection heaters.
Mysa will not work in low-voltage homes, and Nest will not work in high-voltage homes.