Hetzner gives you bare (or virtual) metal that you have to install stuff on, and maintain it.

Fly.io and similar give you a higher abstraction layer where you throw a Docker container and the stuff below and around it is managed for you. Basic suff like OS patches, but also autoscaling, load balancing, request routing, etc.

Does the 4x difference make it up? It will depend entirely on you and how good you are with the things fly.io abstract for you, and how much you value your time spent on it.

If you are literally just looking for a place to cost-effectively park a VM, you should very seriously consider Hetzner! It would be weird if there weren't use cases where Hetzner comes out ahead.

HN always mentions Hetzner, but they're no longer an exceptionally cheap VPS provider. They're basically the same price as OVH.

I forget when exactly they raised their prices, but it was several years ago.

The big benefit of fly is that your server only costs money when it’s doing something (with autoscaling enabled).