As of right now, apart from offering a unit type of every class in every single era, which unless the tech tree were expanded significantly, would be ludicrous, they almost perfectly cascade unit type/class upgrades across all eras so that you're getting units of similar types but of different classes in every era. The unit classes are;
Melee, Ranged, Anti-cavalry, Heavy Cavalry, Light Cavalry, Siege, Naval Melee, Naval Ranged, Naval Raider, Air Fighter, Air Bomber, and finally Support - which are all one class and offer a bunch of different perks.
This essentially translate to two different classes of every unit "type" - Two ranged units, Two Mobile units, Two melee units, two Air units and [Three] Naval units.
Each unit
class usually lasts two eras, while a new unit of the same
type is added every era. I see little reason to change it. You can see the upgrade path by class and era below, and you can notice how it usually cascades with units of similar types;
Notice how the Ranged/Siege lines cascade perfectly every two eras (discounting slinger). The only breaks in this pattern is that the AT units comes an era late, which is fine when you consider that so do both the Light and Heavy cavalry upgrades past the medieval era. The Caravel also finds itself back one era, but appropriately in my mind, the renaissance also adds raider ships - the pattern of two era usability continues after this . Planes are obviously exempt from this pattern as they come at the very late game.
So really, observing the pattern and looking at the numbers -There are 4 of every major unit class apart from planes and raiders, not including start units. So the only unit that is "missing" is 1 Horse unit. Specifically, an additional light cavalry. But I find the skip a reasonable one in order to push back the pattern in preparation for the modern units.
I do agree the gaps seem very big between units. It has the unfortunate effect of making unit upgrades very powerful. We have seen in several videos how people can crush everything with a couple of Crossbows if they get them early, which is not exactly good for balance. There was a reason why the Composite Bowman was introduced back in Civ5, go figure ...
I don't think anything about the footage we've seen can be taken as an accurate reflection of game balance. That said, I disagree with your assessment as I watched one video where a handful of Field Cannons went up against a handful of knights and nearly lost. But ranged units have always been inherently more powerful in the hands of a player over an A.I.
