Biggest drawback of post-via-email: no success/failure indicators. You have to take the extra step of checking the site to actually confirm that your post made it through, and that it ended up where you wanted it to.
Without the use of an app like Drafts to help me mediate the sending of these emails, I'd probably be too concerned about leaving a typo in the subject line of one of my emails, thus unintentionally creating a new stream and leaving one of my thoughts orphaned in it.