I feel there are alot of balance issues with the military units. First upgrade cost don't depend on the cost difference between units, mening the cost to upgrade a scout or a horseman to a knight is identical, which open up exploits in which you research technologies in such way that you can build cheap units to upgrade into more expensive ones, upgrade cost is also low at half the industry price of the unit and cost no additional population.

Ancient era units seems relative balanced, scouts are cheap but weak, scout riders are faster scouts with maybe a little too much combat strength. Warriors are cheap resourcesless melee units who can be upgraded into strong swordsman, spearmen seems to dominate the ancient era but fall behind in classical. Archers give ranged support and chariots can be used for rear attacks.

Classical era see swordsman who are stronger than spearmen without any cost increase which may be a problem and horsemen is a mobile cavalry unit that is stronger than any other generic unit but like chariots are kept in check by its cost and the fact it is tied to a weak leaf tech. Main issue with classical era is simply the lack of generic units.

Medieval era have greater issues. Pikemen and crossbowmen are the first units in the tech tree, need no resources and have low industry cost compared to other units in medieval era. Great Swordsmen cost twice as much as pikemen, need resource for only 4 combat strength stronger. In comparison swordsman cost the same as spearmen but are 5 combat strength stronger. Knights are maybe even worse, at 800 industry they cost 4 times as much as pikemen for 5 combat strength more. In comparison horsemen have 6 combat strength more than spearmen for only twice the industry cost. Thus great swordsmen and knights are overpriced compared to the pikemen. Knights also have the issue that they don't have a upgrade in early modern and given they don't even shine in their own era make them look like a terrible investment. Saboteur also seems overpriced for a unit with similar strength as a pikeman.

Early modern have some serious issue with the musketeers and man o war being stronger in terms of a cost basis than many industrial era units. Musketeers upgrade into line infantry which only have one more strength but cost 2.5 times in terms of industry and twice the population, line infantry is not an upgrade but a downgrade from musketeers and if you can build musketeers you should avoid line infantry technology. Man o war seems to be stronger than the industrial era ships, except ironclad. Steam frigate in particular look useless, being weaker but not faster than the man o war and ironclad.

Industrial era is filled with problems Partisans seems to be superior to line infantry given they cost half as much and have stealth for only two combat strength less. Howitzers are a strong upgrade from mortars, siege artillery is a weak upgrade in terms of combat strength but add the bombard ability which may be enough to consider this unit, however it wont upgrade in contemporary except for chinese. The heavy machine gun seems like a bad unit, at 50 strength it is just 4 strength stronger than the much cheaper musketeers and have no upgrade in contemporary, it may be a unit that is obsolete as soon as you can build them. Dragoons also seems overpriced for their combat strength and while they have some mobility, the question is how good that is in an era filled with ranged units.

Overall here are some points:

• Upgrade cost should be tied into the cost difference between the units and should probably also take account of population. Upgrading from cheap units into expensive ones should not cost the same as upgrading from expensive units to expensive units.
• Swordsmen could maybe be somewhat more expensive, they costing the same as warriors and spearmen while being so much stronger is a problem.
• Horseback riding tech need to be more important/valuable, it is pretty bad right now which make it hard to justify researching it and swordsmen are nearly as strong as horsemen while being half the price.
• Great Swordsmen, saboture and knights need to cost less or in other ways be made better compared to pikemen.
• The flintlock tech units, musketeers and man o war need a nerf or industrial era units need to be stronger.
• Heavy machine gun could upgrade into the All Terrain PMV or its generic equivalent.
• Dragoon could be made stronger or cheaper, same with line infantry.
• Steam frigate and maybe also torpedo boat need a buff, ironclad costal bonus may need a nerf.