Hi primarily yep it is developed on UAE, well not so much developed, just assembled in UAE. I'm using an editor on the PC for the actual programming. When I want to test it I boot up UAE which loads in the source file and more or less assembles it in devpac automatically for me. It is also regularly tested on my A4000, I've set it up to copy over the binary from the pc automatically and run it. The graphics are worked on in a combination of Grafx2 on PC and PPaint/Dpaint on the A4000/UAE, as well as a windows app I wrote to help my SAM Coupé graphics work (updated to provide suitable masks for example for use by the blitter).
Developing on the Amiga alone would be a pain in the arse, especially as the Amiga takes longer to boot after I've crashed it. UAE takes about 3 seconds