this is one of the things that hit me the first time i saw this site and my thoughts are this:
it seems like there may be civ specific units

but if they implement one for every civ it could get like AoE when selecting a civ

.
i thought maybe time related as was suggested but then i thought about the fighters - dont the f-15 and the mig rival each other? and wasnt the abrham tank around since the end of wwII? i might be wrong, about that. but even if i am - they follow quite a short (and less signifigant) upgrade that what has been seen in civ's in the past. i.e. prop plane -> jet fighter -> stealth. each of these were brought about by new techs. what serious tech difference is there between a abrahm (original model) and a panzer (late model), or between a mig and an f-15 (one for aircraft carrier, one is not? that would be too much).
so, i believe that it may be more civ specific

- and just hope that it doesnt get TOO civ specific like AoE (took me for ever at first to figure strategy, and hey, i am an indecisive person - i could never figure out which units i wanted - cause i wanted them all

)
i'm sure they will balance it well, though.