Structural Foam for chassis stiffening

As a sound deadener or as an insulator, it seems like a viable option.
Regarding the structural characteristics, look at your refrigerator or freezer door. If it's less than 8 years old, likely it's made from .028" or thinner metal and then filled with foam. The fridge cabinet is likely .017" or thinner. It's very rigid and light weight.
The issue is the degree of engineering used to calculate precisely how the foam expands into the cavity and what the rate of expansion is. It's possible to over fill a cavity, and the expansion to bust at the seams.
While the chemicals and application may seem benign, the fact is that chemicals such as cyclopentane are extremely volatile.
My real world experience is that I sell over 90000 tons of steel a year to appliance manufacturers. That being said, I am in an appliance factory nearly every day and work with the engineers and manufacturing people who are using foam everyday. The design of the cavity, the flow and expansion rates are highly engineered and the chemicals can be dangerous. Adhesion rates are also carefully crafted and monitored.
There is likely wrong with using a bottled premixed product to insulate for heat, sound, vibration etc. My bet is the auto companies have put considerable time and resources into designing the application and end use to achieve desired results. Likely those techniques are not available to us as a viable and cost effective measure.
Just my opinion.