May be unable to create elemental forces, being limited to shaping from already existing sources.
Platforms structural strength may be limited by the users will.
In most cases, unless the platforms are made of solid matter, they will return back into their original state if the User becomes unconscious, leaves the proximity or otherwise loses contact/control of the item.
If platforms are not supported they'll also drop.
May need to be attached to some surface.
Crystal (Marvel Comics)
Airbenders, Earthbenders and Waterbenders (Avatar: The Last Airbender/The Legend of Korra)
All Avatars (Avatar: The Last Airbender/The Legend of Korra)