I think it's reasonable to escalate criticism of Heroku for remaining in a single AZ. They have had plenty of time and resources to fix this, and haven't, despite being quite competent. I don't know if it is that they don't think it's necessary (due to the profile of their current customers) or what, but I wouldn't use Heroku for anything as long as they remain in a single AZ, and would be really reluctant to advise other people to do so. I obviously really like the Heroku team and product and would love to use them otherwise.
It wouldn't even need to be true seamless failover across AZs right away -- just offering a us-west and us-east Heroku would be enough for me, with shared nothing (maybe billing, or not even that), and then figure out redundancy yourself inside your app. Multiple regions is WAY better than multiple AZs within a region, too -- both for reliability and for locality.
Obviously a real seamless multi AZ/multi region solution would be much more technically impressive, useful to users, and Heroku-like, but they shouldn't let the perfect be the enemy of the good here.