Mostly I’ve been surprised that people considered Heroku as an option for serious projects.

• If Heroku needs to upgrade Postgres or similar your application goes down for 10 minutes. No chance to do your own a DB pivot.
• You can’t control the routing layer or customize your haproxy or nginx configuration. So you’re tied to whatever Heroku thinks your buffer size, your request timeout, etc etc should be
• It gets seriously expensive if your app gets any kind of traffic.
• Not to mention the occasional outages/deployment failures mentioned in the article above.