I'm finding lots of patterns/blueprints on my tribes land now. Not sure if that is intended.

The biggest problem I see with rares is utility. Your odds for finding rares are about 1 in 6000 actions in an extreme zone with max skill and unknown stats. But of those results, how many were actually useful? Right now only armor gets bonuses, but even if weapons and tools get bonuses at some point, the majority of rare finds will be studs/boards/poles/etc. I doubt that buildings will ever give bonuses, and even if they do no one will be able to find enough studs and boards and shlep them back home from an extreme danger zone to build a gold dome over their junk pile.

What are the chanced for a rare in low, medium and high danger zones?

I do like the chances on finding quality above and below my modified skill. Gives a chance of a nice reward (the occasional supreme) that keeps me scavenging.