What makes a growing medium good?

With the thread about peat moss and alternatives like coconut coir, I got to thinking about the byproducts from things in my own gardening and kitchen. In particular, I’ve got a unique byproduct that I would love to suss out to see if it’s valuable or not, but I’ve got no clue how to actually measure the efficacy of it. I understand that it’s about moisture retention (or lack of), air/space, etc, but I’m curious how others have figured out if something you use is good or not.

The byproduct for me in particular is spent chaga, which has been ground into a coarse grind and steeped for over a day so no more colour is coming out of it. It really intrigues me because I think it might be a super-medium but I don’t know how to tell beyond just growing seeds in it and seeing