That's only a concern if you care about how many people read your blog, but if you do, it's a concern.
Medium's also very readable. If you choose your own theme for your blog, the chances are you'll hit a theme that's "less readable". Often, programming blogs are a LOT less readable. So that's worth considering too.
(All other points made elsewhere about control of your content, compatibility with FOSS philosophy, etc, are good ones too and I won't repeat them here.)
Cross-posting to both a self-hosted blog and Medium isn't a bad solution. Or use a custom domain, as suggested.