The Float Stones cuts a Pokemon's weight in half, therefore reducing the damage caused by weight based moves such as Low Kick, Heavy Slam, and Grass Knot.
However, the move Heat Crash does more damage if a Pokemon's weight is lower, so that is one disadvantage. But you could have a different Pokemon use Trick or Switcheroo then switch Pokemon and have them use Heat Crash to deal more damage.
Hope I helped :)
