Ello is a recent app that had to scale to millions of users extremely quickly and did on Heroku. Read their technical write up: http://mikepackdev.com/blog_posts/40-5-early-lessons-from-ra...
Ello had issues, and Heroku would be considered amateur for what they were doing, but if you read it Heroku was not the cause of their performance issues -- their application code was. IMO most business applications run into problems due to performance issues within their application code, not because of server downtime.
Regarding how common downtime on Heroku is... yes, Heroku has lots of minor incidents impairing its toolkit and some services, but these are not issues that go notices. Two years in I still haven't had significant downtime from Heroku itself (more than 5 minutes) during business hours where it matters. Cloudflare, and other DNS providers, have frankly given me more trouble.
Unfortunately, Heroku is down constantly. Don't take my word for it; google "Heroku down" and see how often it occurs.
Microsites? Sure. Hobby sites? Sure. Business applications that generate revenue? Good luck.