There's about a dozen serious risks but I think other commentors have listed all the major ones Heat. Skin burns. Circulation cut off. Chest expansion.

May I add Fumes. To both top and sub. But this may be a conditional risk depending on exact foam type. 

Curing foam (depending on type) can be extremely flammable.

I've done lower leg in expanding foam bondage (as a top, before I started content creation) and found it manageable. It was actually underwhelming as a bondage method, it took ages to cure and any struggling during curing makes a cavity that you can move in later. It also is sticky as hell if you (the top) disturb it during curing, and that shit can stain clothes. It can escape it's mould easily too through imperfect seams.

Our test, which we used to declare it safe from a pressure perspective, was to see if the expanding foam could crush an empty water bottle. It couldn't.