The only problems will be in bombardments, a bombard strength of 1 has very little chance of destroying improvements, while a bombard strength of 10 has a good chance of at least doing some damage, and then you look at the fact that even catapult will have a bombard value of 40!?!? Even in the ancient age you could reduce your oponenets cities to rubble on moments.
I find a much more playable alternative (And one that fosters a mor realistic obselesence factor- no spearmen killing tanks) is to have a HP bonus each era, or half era, to represent the improved armour and battle tactics over the years.
I start with ancient units having -2hps, this gives regular units 1 hp, and elite units 3 hps (notice how this means that in the ancient era experience has a much more exagerated effect, using veteran unit can double thier hps!) In the middle ages all units up to musketmen and cavalry have -1 Hp (gunpowder equiped troops have +0 to represent the impressive armour penetration of even weak firearms) and in the modern and industrial age all Infantry units have a +1 hp bonus (this gives you a reason for using infantry rather than 100% mechanized armour and allows for tacticians to use helicopters.
With a seperation by hps, two units can have identical ADM stats, but be in different eras, A good example is the Phalanx and the pike man, both 1/3/1, but the pikeman has a HP bonus to represent its improved armour.
You can give other units a special HP bonus or penalty to represent numerical superiority or its lack. A tiger tank could have better stats than a normal tank but a -1 HP penalty to represent the fact that they were few in numbers, if you want to get the full benifit, put them in an army. Same goes for the early units.
Ive playtested this extensively and it works great, the problem would be that experienced players may find it easier to beat the A.I. in the early eras, (the A.I continues its tactic of thowing its units at you peacemeal despite the fact that units with 1 hp fare very badly when attacking units with more than one HP, as you may have seen already fom your games, but once the A.I. gets veteran units- either through the war academy of just general production of baracks, combats between units of the smae experience can be very hard to divine, because of the high chance factor) but for less experienced players- having the game get harder as it goes on could be more fun- and you may have trouble killing musket men with your knights, (though that is very realistic) I'm working on some pre-industrial cavalry (dagoons) to fill the gap, wih higher attack and lower defence than knights, the dragoons would also be good for civs lacking Saltpeter.
One off shoot is that micro management of wars is not so tiring as combats a quicker and more decisive.
Also there are many ways to add caracter to a unit, give it a Zone of control for example and it is instantly better than a unit of same stats but without ZOC, and a defensive bombardment is also very usefull, though I recomend a value at least half its normal bombard rating.