There's a lot of chat about fly.io on HN. possibly just because the founders and friends are posters here.
Is there any in depth comparison between them and render.com ?
Not in-depth, but two things that have stood out to me in assessing Render vs Fly are Render's lack of Multi-region apps and release phase scripts for running migrations. Easy multi-region apps seem to be a main selling point of Fly. However, both of the features I mentioned are on Render's roadmap (Multi-region apps has been on their roadmap since 2019, so maybe not a priority?)