A minimum of 1 Bar of pressure up to a maximum of 3 Bar is required to function a steam shower. As a combi boiler is a constant and stable pressured system they are the ideal water supply for a steam shower.
Mega flow systems or any other pressurised systems are suitable. A system that will require attention would be an 'open-top, gravity-fed system'. This type of setup would require a 'booster pump'. Please always consult a professional in regards to your individual setup if uncertain