Note: I always play on huge maps, on Emperor or Immortal level, und using fractal map (or occasionally archipelago for variety and naval warfare). These comments may not apply to people playing different styles of game.
In Civ V there are two unit types which may as well not exist: the Spearman / Pikeman / Lancer / Anti-tank / Helicopter Gunship progression, and air units (fighters and bombers). These are always weak, and not worth the cost of building or promoting. Additionally, the foot soldiers (swordsmen through mechanised infantry) are useful defensively, but never for offensive war, so become redundant if/when you become strong enough to no longer fear invasion..
There is also the problem that from the early Mediaeval era through to the atomic era all available units are too weak to successfully carry out offensive warfare at these difficulty levels. There are effectively only two periods when effective offence is possible: the Classical era using composite bowmen / sworsdmen / horsemen, or occasionally the Mediaeval using crossbowmen / longswordsmen / knights. At this point the window closes until the late atomic or information era, when rocket artillery / mobile SAMs / modern armor / XCOM are (properly wielded) always successful. The implication of this, therefore, is that the size of your empire - whatever victory type you are aiming for - is totally fixed from about 1AD, unless you manage to find a useful uncontested piece of offshore real estate (pretty rare).
In Civ III, despite its totally different combat mechanics and units, similar considerations applied.
I haven't seen much about Civ VI combat. I fully expect the AI to remain reliant on spawning lots of units rather than using brains, but I really hope there are no more useless forces, nor the long period where no offensive warfare is possible.
In Civ V there are two unit types which may as well not exist: the Spearman / Pikeman / Lancer / Anti-tank / Helicopter Gunship progression, and air units (fighters and bombers). These are always weak, and not worth the cost of building or promoting. Additionally, the foot soldiers (swordsmen through mechanised infantry) are useful defensively, but never for offensive war, so become redundant if/when you become strong enough to no longer fear invasion..
There is also the problem that from the early Mediaeval era through to the atomic era all available units are too weak to successfully carry out offensive warfare at these difficulty levels. There are effectively only two periods when effective offence is possible: the Classical era using composite bowmen / sworsdmen / horsemen, or occasionally the Mediaeval using crossbowmen / longswordsmen / knights. At this point the window closes until the late atomic or information era, when rocket artillery / mobile SAMs / modern armor / XCOM are (properly wielded) always successful. The implication of this, therefore, is that the size of your empire - whatever victory type you are aiming for - is totally fixed from about 1AD, unless you manage to find a useful uncontested piece of offshore real estate (pretty rare).
In Civ III, despite its totally different combat mechanics and units, similar considerations applied.
I haven't seen much about Civ VI combat. I fully expect the AI to remain reliant on spawning lots of units rather than using brains, but I really hope there are no more useless forces, nor the long period where no offensive warfare is possible.