But you gain the downside that if your password database gets leaked, now every users' password is there in plaintext. All you have to do is bypass the hashing function in the client-side login so you can send whatever hash you have in your database leak.

You could hash them again before they hit db, but then your not really pre-hashing the passwords clientside. And if you want to change your hashing algorithm or parameters, users can't login until they update their clients.