Never used Fly.io but I have a website running on Render (2.4M unique users per month) and the service is getting quite nice. Occasional hiccups, but they are improving. Not as stable as Heroku used to be 7-8 years ago yet, but it's also waaaaay cheaper.

I really like their Docker support and infra as code, makes it very easy to spin up a whole thing while not being too far conceptually from Kubernetes for example.