I just realized that in all my time within the Nether I’ve never had a mob spawn in my netherwart farm. Now, while this would make sense after my farm was complete, because of the netherwart being on the block, it took my quite a bit of time to fill up all my blocks with netherwart. And in all that time (3-5 hours) I never got a single zombie pigman in my farm.
Can Zombie Pigmen spawn on soulsand?
Mobs definitely do spawn on soul sand, the solid block requirement is for the block graphically, and soul sand graphically is a solid block (no transparency).
I did test it with a glass enclosed room with a soul sand floor and zombie pigmen did spawn inside.