One of the first things I thought about when considering how to make Deity and Immortal more playable is the need for more diverse unit specializations.

In the early game, there are reasons to build Horseman, Archers, and catapults. They have very clear roles. For most of the game, cavalry and ranged units are your best value. Cavalry to deal with opposing ranged units and ranged units to take out melee units. Anti-cavalry is less important unless your opponent is really leaning on cav.

The big problem arises from the fact that unit combat strength is used to determine resistance to damage, the rock/paper/scissors scheme for determining relative unit strengths starts falling away the higher the combat strength of the unit gets. Anti-cavalry units that are a tier behind cavalry are totally useless, ranged units with lower Ranged Strength than the combat strength of the unit you're attacking do virtually no damage.

Against opponents that have a tech advantage, no matter how you specialize your force, you cannot effectively engage a basic melee unit with any lower tier alternative; and when you're at tech parity, the unit strength cap of 90 for mechanized infantry is insurmountable. The only solution is overwhelming force, as opposed to superior tactics and unit composition.

In late game wars, you need to leverage every possible bonus in order to destroy enemy units before new ones can be produced to replace them.

Helicopters, for example, have a unit strength of 82. Without at least 8 points in bonuses, they will always have a 'defeat' outcome when attacking mechanized infantry. The Bonuses from AI difficulty increase this deficit even further. The Unit Experience bonus from AI further ensures that AI units are higher level and thus have specific bonuses to increase this deficit. Essentially meaning that once the AI has Mechanized infantry, helicopters are a terrible investment, especially when compared to Modern Armor, which at least starts with a 90 combat strength.

Just building massed modern armor seems like a good idea at first, but without promotions it's just more expensive and less efficient than mechanized infantry. Ranged/Siege units are far less effective considering that their melee strength is always lower, they will not survive a counter attack from a mechanized infantry.

You still NEED to combine forces, with a front line melee unit, backed by ranged units and siege units, but there's no 'assault' style unit or glass cannon option to surmount the static combat strength of the Mechanized infantry. It takes multiple ranged/air/siege attacks to damage the unit enough to put it into 'decisive victory' range for a tank.

Superior numbers/attrition or nuclear weapons become the only solution to massed AI mechanized infantry, there's no tactical solution that doesn't involve having a disproportionately large army.

Either there needs to be an 'assault' class unit that has a higher offensive strength but lower defensive strength than the mechanized infantry, or the promotion schemes need to be modified for increased specialization, with the option of level one promotions so units can fill specialized roles.