Minecraft wiki entry for Best_biomes_for_homes, associated text: Crowded — This can make biomes difficult to move around in smoothly, as well as requiring the player to landscape the environment if they intend to build. Biomes that contain an excessively large amount of trees can shelter monsters during the daytime and allow them to resume spawning under the cover of vegetation. Because of this, players can potentially get ambushed by groups of monsters at a time. Biomes that contain a lot of hills and mountains can cause similar ambush situations.