Minecraft wiki entry for Hunger_management, associated text: Disadvantages:
Like other stews, mushroom stew does not stack, so crafting a lot of stew at once takes up a lot of inventory space.
A bowl of stew requires both types of mushrooms, which is bad news if you only have one type.
Mycelium, podzol and nylium aren't easy to find, the first two found in rare biomes, the third only found in the Nether.
Even if these blocks are found, the player can't obtain them without Silk Touch.
Spreading based mushroom farms are slow.
Huge mushroom farming requires bone meal, which is hard to obtain in large quantities in early game.