I think I figured it out. Played vs 3 other substrate on challenging. All 3 got stuck.
This time I played as substrate too and did exactly what they did.
You start with 100 rads, the factory on the ui panel says 45 rads to build. If you build one, it actually consumes 90 rads, 2x what it should.
When the ai tries to build 2 assembly bldg, it run out of rads and has no means to build and capture more rad income.
If the ai only builds one factory it will have 10 rads left as long as it doesn't build rad requiring stuff it can build it's first army.
So the problem seems to be not with the ai, but with construction and using more resources than the ui says it should.
Tried building a listening post as phc, I think the cost is 80 rads, I had 100, but it didn't finish I ran out of rads, so looks like the issue is really building is using more rads that the ui is displaying...