you cant put a unit that has unload capability into another unit that also has unload capability....air or ground or sea...
so..
if u made a mech inf able to load ppl, you cant put it into a transport ship or a plane that can load stuff
by same token, you cant put a plane that can load ppl into a carrier....
thats prob done so you cant transport a gazillion units with only 1 unit by stacking them up