Think of two libraries that use unsafe Rust and interact with the same hardware, but work correctly when used on their own.

A program written only in pure not-unsafe Rust might use these two libraries in a way that breaks because the assertions the programmers of the libaries had, like for example having exclusive access to the hardware, are wrong now.

One could argue the pure not-unsafe Rust program is wrong, not the libraries.

I think klodolph's comment is very thoughtful and shows a good deal of experience and domain knowledge.