Ship style has benefits that got both ways, some prefer Aero for the smaller damage to the ground they do & atmospheric travel ability. So I was looking for something above & beyond that arguable advantage.
However, in a military operation, the spheroid dropship HAS a lot of advantage over the Aero --- it doesn't need a runway to land, so you don't have to capture an airstrip to bring the ship down to either unload, or pick up for an evacuation.
With a spheroid, you can more or less pick any level spot on the planet, tell your mechs to meet you there, and drop in and get them out.... with the Aero, you need to have them hold an airstrip long enough to land, load, and then take off again, and a single artillery round making a crater in the runway means you go nowhere.
That's more than enough reason for picking the Union, even if the Buccaneer had been a better design.
Nahuris