Yeah, I see it from both sides. I lean towards being an independent developer doing a lot of prototyping so I like Fly's tolerance of quicker, less strict practices in this regard. Not had any problems with fly.toml so far, but my apps are relatively simple.
We tried[0] to find Heroku alternative a couple of weeks ago. In short Render.com feels solid and Railway.app very promising, hope they will do more progress soon.
I migrated a static site and a few apis off heroku onto render.com a few months ago and the experience has been really great. Fills the niche of Heroku perfectly for my use cases.
We tested Zulip not too long ago, in short - UI is not so good, but in general still useful: https://nixsanctuary.com/zulip-review-through-slack-glasses/