Michael,

Thanks for looking into this even further.

I tried as you suggested. Indeed, splitting the edges seem to help a little, but even the simple thickness seems to fail under a certain number (about 1mm in the example, and it does generate parasite surfaces on the rims that do carry over to the rest when the groove and separate height options come into play. I tried again with a fresh cylinder and oval, but it gets the same weird things happening.

With a clean smooth surface and simple shape to inset, there are easy other ways to achieve what I want, but when it comes to complex shapes like rudders or airbrakes on tailplanes and wings of aircraft, it would really help to have an automatic inset working there, but I understand that it is probably not that easy to get.

Nonetheless, in those cases when it works I am confident that the Inset command will help me a lot.