Platform racks are heavier to begin with.
Sway is dependent on how it locks with the hitch receiver. The ones that have a plate or ball pushing onto the inside of the reciever, pioneered by 1UP, significantly reduces sway within the reciever.
The higher end platforms don't touch the bike frames, and secured by the tires (1UP, Yakima Holdup, Thule T2, etc).
Hanging requires straps that touch the bike frame and the front wheel can rotate around, unless secured with a strap or bungie cord. The rubber straps can be a PITA to put on and remove. Yakima ratchet straps... make sure you don't lose them.
Acess to trunk... platforms tend to rotate down. Hanging ones, you would have to buy one that swings out. Since mine, and many others, you have to unload the bikes to swing down the mast.