Solids are ok, I think of them as the large brushes with small poly count, the tiny smaller adds are for semi solids.
Since the whole thing is dependent on "adding" a brush inside a subtract which removes the outside walls you never see or not rendered.
however semisolids actually DO interfere with the subtracts, it isn't good to use those as a big add brush just to fill space. A 10 poly brush has no reason to save nodes.
So I am thinking semisolids are better utilized as an add brush for things to finish a map, not so much as making the main structure (other than steps-stairs)
Congrats on the green, you deserve it.