Minecraft wiki entry for Best_biomes_for_homes, associated text: Trees are a bit uncommon, although the plains biome often has forests nearby.
There are many open caves which can disrupt the flat landscape, and mobs such as zombies can come out of the caves to attack you.
When in combat the tall grass can often obstruct your melee attacks, while hostile mobs are not blocked by it.
Skeletons and witches can more easily shoot you down due to the lack of obstruction.
Tall grass can clutter your inventory with seeds, and can be annoying to clear out when building a large structure.
Lava lakes generate here, which can easily kill a player or animal and will destroy any items dropped.
Pillager outposts can generate in the biome, and although it has loot chests, it spawns pillagers infinitely around the structure and can easily kill unarmed players.
On servers, a base here will be very easy to see and therefore may become a prime target for attack.