Heroku isn't for apps that can't stand downtime. My experience has been that if you have 2-3 heroku apps, and you monitor them with a 3rd party tool, you'll see random "server not found" behavior every few weeks. (And no, they're not just timing out from dyno spinup). Usually this isn't a system wide outage and never gets mentioned on their status page.

So only use heroku if:

a) Uptime is non-critical & you just don't want to deal with setting up a server b) Uptime is non-critical & You don't know how to set up a server