If you can steal the password hash database, stealing the shared "salt" is usually going to be trivial. Second, lots of hash functions are vulnerable to length

If you can steal the password hash database, stealing the shared "salt" is usually going to be trivial. Second, lots of hash functions are vulnerable to length extension attacks, so even if extracting the shared secret directly is hard, you can probably exploit the system to see what an empty password hashes to and then use that as a basis for brute-forcing the rest. In other words: there is a reason why security experts and cryptographers recommend the password storage mechanisms they do.