Yes, the Yellow Comet units look very outdated, with spoke wheels on their vehicles, and they're the only army with a propellor plane! ;o) But they can be very strong, especially under Kanbei of course.
I think the Green Earth units are meant to look a little bit like European World War II units, although it is really a mix and match of other units too.
The planes and copters of the Orange Star army always made me think they were meant to be the Americans, and Blue Moon was Russia.
But I think it isn't 100% accurate, because all the armies have units that are actually from other countries too. ;o)