I really like that the Extreme economy is very adaptable to different scenarios. I usually play with Extreme, but don't use all industries it provides in all games. I really like that all industries are available, but not indispensable. So everything works even though not all industries are used.
Also I really like generalised cargo! IHMO the Extreme economy should be complex in terms of industries, but not too complex nor too simplistic with cargo. I prefer cargo which serve relatively many purposes, i.e. farm supplies / engineering supplies.
Therefore I'm really fond of the newly added packaging cargo
Things I don't really like, or didn't use that much are building materials and their corresponding industries. But I didn't play that much with FIRS 3.0 yet, so it might turn out rather well by now. Still, I can't put much sense into the hardware store industry, as there's already the builders yard.
About clay, why not a pottery industry?
(produces goods, packaging)
Btw, what I'd really like to see in FIRS, too, is tourism!
Though, that might serve a purpose for an own economy