Minecraft wiki entry for Best_biomes_for_homes, associated text: Aside from mooshrooms, no other passive mobs spawn which makes obtaining food harder. Although, you can just milk the mooshrooms, which may or may not be enough food.
No natural light-based mob spawning means creating a mob farm without a spawner is impossible or without transporting animals and breeding them. This means the only way to obtain drops such as bones, string (to craft bows), or arrows (for use in bows) is to build out over the ocean.
Alternatively, if you have a composter, you can convert mushroom into bone meal.
Make sure to have loads of emeralds as wandering trader trades are very useful in these biomes.
The lack of wood, which means no first set of tools or crafting table or anything crafted with cobblestone or wood. No wood also means no bowls, which makes obtaining mushroom stew harder.
The only wood source is from shipwrecks which can generate on land in mushroom field shore, although they are quite rare.
Rarely connected to any other land, and if you spawned here for the first time, you will have a very hard time going to the mainland due to your hunger going down from swimming. You can't make a boat either, because wood is not available in mushroom fields. Unless you somehow swim to the mainland, the biome is connected to other non-ocean biomes (very rare), or you find a shipwreck, wood is inaccessible.
If you spawn here, you must swim back (or go back) to the mainland (which can sometimes be thousands of blocks away), find a shipwreck, or go into the middle of the sea to fight patrols to get emeralds to buy saplings in order to get wood. If the mainland is too far away, you should consider creating another world.
The whole island is covered in mycelium, which makes it's not a good place for big builds.