I think some sort of "linking" is probably the best way to avoid super-units.
It's hard to imagine, for example, a real life army having 10/10 tanks but only 6/6 Infantry and 6/6 artillery! As you say, the units are actually more combined arms than "pure" units. So how do the tanks get 10 attack guns, but artillery lags behind at 6??
Linking units, say tanks, arty and Inf, and not letting them get too far apart would limit research in a realistic way. Say 2 levels diff max, or 5 combined on evasion/attack. Therefor to get 10/10 tanks you'd need 8/8 infantry AND 8/8 artillery, or 8/7 Inf and 8/7 arty (or some other combination). This would reflect the inherent combined arms represented in the game.
Same for air and sea units, to some degree. No 9 ground attack Heavy Bombers with 5 ground attack Tactical bombers. An exception could be made for the Atomic Bomb.
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No Law but the Laws You make