[eagletusk]

Most of the nutrients in mushrooms are behind a cell wall of chitin, which is a fibrous substance consisting of polysaccharides and forming the major constituent in the exoskeleton of arthropods and the cell walls of fungi.

because of this chitin, eating a mushroom raw is not very nutritious and you miss out on most of the health benefits.

[mattnewport]

Not being nutritious and making you sick are rather different though. Although porcini can be eaten raw it is probably most commonly fried or sautéed. Boiling it seems like a bit of a waste of a food prized for its flavor.