It's definitely not unusably unstable, at least from my PoV, and in fact, I haven't really been hit by most of the instability personally; or at least, if I have, it has gone mostly unnoticed by me. I had an issue where I couldn't deploy for a while, but the services that were up seemed fine. (Disclaimer: I do not use the Redis/database features.)
That said, the point I'm trying to make is that it is in fact a lot nicer to use than simply YOLOing whatever onto servers. This model of PaaS is basically what App Engine should have been. It's difficult to want to go back once you've used it.
