I think Jeff answered the question pretty much as well as it can be answered. Strength is a matter of construction details like the rim extrusion shape/size/thickness, number and type of spokes, etc. So is lightness. A wheel builder balances those factors (and others including cost) when creating a wheel for a specific application, whether it be a "road" application or "mountain".

Do you need road wheels built for racing or touring? They're not going to be the same even though they're both road wheels. Will you be using your MTB for cross country, downhill, or dirt jumping? You could opt for a lighter, more nimble wheel or a stronger wheel depending on your answer.

As for cyclocross wheels, since you asked... They come in various spoke counts (mine are 24 front & rear) to balance strength and lightness to the rider's needs. They're not necessarily stronger than other road wheels, but they are generally a bit wider than racing-style road rims to better accommodate fatter tires for off-road use.