I find render.com basically as good as Heroku and certainly much better than fly.io's unpredictable pricing

In 2022 Render increased their prices (which for my team worked out at a doubling of costs) with a one month notice period and the CEO's response to me when I asked him if he thought that was a fair notice period was that it was operationally necessary and he was sorry to see us go.

In what way is Fly's pricing unpredictable ?

It varies based on usage

Isn't that how things should be ?

From a consumer's standpoint, why would I choose that?

It's a natural law to pay based on what you consume. Whenever that isn't required, you're typically being subsidized in a form or another, but don't be surprised when it goes away.