The problem is not so much "when unsafe code gets called indirectly by safe code", which is fine if the unsafe happens to be sound. The problem is that "when C-like or Zig-like unsafe code wants to call safe code", the safe code is running in a context where its implied invariants may be violated, leading to insta-UB. Hence code that's intended to provide "library-like" facilities to unsafe blocks cannot be idiomatic Safe Rust, and it needs to be written even more carefully.

For instance, any &mut reference in Rust is assumed not to be aliased, and any &reference not involving UnsafeCell<...> is assumed not to be written to. These implied contracts can be loosened, e.g. by using &Cell<> if applicable (may alias, can be read or written safely, but only "as a whole object": access to the internals does not escape beyond any single operation) which is arguably closer to idiomatic C.

MaybeUninit<> is another common example: C and Zig code often works with possibly-uninitialized data, but this possibility has to be accounted for explicitly in a safe Rust interface. It's always insta-UB if a safe Rust function is passed uninitialized data, even when the equivalent would work idiomatically in C.