Per the wiki (understood not official) mushrooms are not spawning at near the advertised rate.

Per the wiki mushrooms have a 1% chance per tic of spreading.  In my test (creative) superflat world that I use to test new mechanisms I built an automated mushroom farm.  This has a 3x3 arrangement of podzol with a single brown mushroom in the center block with observers looking down at the other 8 blocks and wired via redstone to auto harvest whenever a mushroom pops unto any adjacent block via hopper minecart running underneath.  In 24 hours it has produced a total of four mushrooms.  Please do not make me dig out my statistics textbook.