We're having this debate internally. I'm trying to motivate teams currently blogging in Medium to come back to the new CMS we're building for the whole company. Briefly, side A says, Medium is a well-known platform with good discoverability and a great writing experience, and having a popular high-ranked site outside of our core helps SEO. The other argument is mostly, we want our web properties to be more than a brochure, we want the voice of our company to speak "from it" rather than some other random place (even if one presently popular), we should avoid moving between fashionable 3rd party platforms, etc.