What is a disadvantage of speaking your thoughts rather than writing them?

One disadvantage of speaking your thoughts rather than writing them is that you can think through what you'd like to say more clearly. For example, rather than blurting out that your boss's business plan is bad without having a solution to whatever is wrong with it could easily be avoided with writing. With writing, you can address what you find is wrong with the plan and formally propose solutions, all of which in a polite manner.

However, a major downside to writing thoughts is desensitization since you can't read and gauge the expressions and emotions of the person you're speaking to.