I agree with writing. Sometimes it's the only way for other people to just shut their mouths and actually focus upon the message without constantly interrupting you. Sometimes it's the only way we can have a meaningful dialogue without the anxiety of an actual face-to-face.
On the other hand, you do have to be careful with some people because many are used to reading a statement, then trying to "read between the lines". Some people are masters at twisting the message so that THEY are the victim. "So, what you're trying to say is..." Then they put you on your heels trying to defend something you didn't say, at all. (Which I find quite upsetting.).
My advice, just be as direct and blunt as you can. Tell them in your letter, to not read between the lines. Say what you mean. Get to the heart of the matter. They're going to be upset and defensive, regardless. Expect that, but try not to get into defending "something you didn't say at all" situation.