Well, one issue is insurance. I doubt insurance cover would be valid if a nitro cartridge was used in a gun without suitable proof.

I also believe it is more complex than peak pressure, and that rate of pressure rise can also be important.

I know that many BP proved guns will pass (and have passed) nitro, so they are sufficiently strong for the forces. But until that test is done, the risk (both of accident and lack of insurance cover) remains.