Pressure is something we researched heavily. In fact, in our first year, we worked with the National Science Foundation to get an entire test home where we could outfit the heating system with dozens of sensors and push the HVAC to the limit. We learned that we needed to do what most traditional zone damper systems do and add pressure and temperature sensors to the system to ensure that enough hot and cold air was able to flow over the coils.
We then added another layer of safety by designing the vents with pressure bypass dampers that leak air when the pressure builds up so that even if the system is offline, pressure won't be a problem. The vents themselves are designed not to seal closed. The only thing we recommend against is trying to close off every vent in your home at the same time - even with manual vents.