I don't really have a good answer for that. I'm happy with what have at Heroku. My pain points on it would be:

1. The annoying 30 second timeout they have (Which is not there in other platforms)

2. Steep pricing (Which I'm not really aware how comparable it is with Fly)

3. Lack of HTTP/2 support (Which is coming soon to Heroku I think)

4. The feeling of stagnation around Heroku

And Im not sure if resolving all of these would be enough of a incentive for me to make a move.