Yep, I'm happy with Pinta as Paint.NET alternative on macOS.
Sometimes, I feel GIMP is a bit complicated when I typically want simple quick processes like cropping, paintbrush, etc.
this app is almost as old as dotNet and always felt like a showcase. it works, but understand your concerns. especially as the format is exclusive to their app (and the only way to have layers)
- It is not open source.
- It does only run on Windows.
Alternatives include Krita, Pinta and The GIMP.