I just finished a game on insane map size with everything set to rare. Maybe I'm just weird but rare to me means you don't see very many of them, but there are at least some. This, unfortunately, didn't seem to be the case.
I had about half of the galaxy explored and owned about a fifth of the map. I saw about 5 or so sources of thulium, which is about what I'd expect for setting resources to rare. However, I didn't see a single source of elerium or antimatter which is rather concerning on an insane map size, even when the setting is rare. The worst part was the durantium -- there were dozens of them in my territory alone, pretty much a source for every solar system. There seems to be a large disparity amongst the various resources and how they are generated -- it simply makes no sense that there would be a handful of thulium, no elerium or antimatter, but dozens and dozens of sources of durantium. Doesn't really seem to fit the definition of rare and certainly lacks consistency.
I understand that some resources are tied to other things in the galaxy. Durantium needs asteroid fields, elerium needs nebulas, and antimatter needs blackholes. I could see having lots of durantium if I set asteroid fields to abundant and would expect no antimatter if I set blackholes to none -- but I didn't do that, I set everything to rare. Despite that, there was an asteroid field in pretty much every system, hardly what I would consider rare. Nebulas were fairly spread out but there were definitely enough of them that I would have expected at least some elerium. Didn't see a single black hole -- I know I only had half the map explored but still, insane map size and didn't see a single one.
I've played several games in the past with tiny maps and abundant everything, and there always were far more asteroid fields and durantium than the other features and resources. I get that "abundant black holes" probably shouldn't spawn the same number as "abundant asteroid fields", but then you also see multiple sources of antimatter at a single black hole while only one durantium per asteroid field, so it has the potential to equal out.
Overall, there just seems to be way too many asteroid fields and way too much durantium regardless of settings. Hard to determine if the other resources and features are in line with each other, but the asteroid fields and durantium is just crazy. It seems that the asteroid setting really only changes the size of each solar system's field and not the chance of solar systems having them. Perhaps there is a design reason for having slightly more durantium but this is way too much compared to the other resources. Just my thoughts and experiences, anyway.