Why was it only safe to write in metal in Mistborn?

Answer 1

It was too easy for powerful allomancers to alter other substances in ways that changed the meaning or left out certain important facts, in order to establish their own power.

Answer 2

In Mistborn, writing in metal was safe because it could only be read and understood by Allomancers who could burn the metal to decipher the message. This made it a secure way to communicate without non-Allomancers being able to easily intercept and decode the information.