There are quite a few civs that have Unique Units that are good, but only in small numbers. This means that getting all their upgrades is often not worth it. Elephant Archers are a good example of this; you’re probably best with about 5 Elite Elephant Archers, but with that many, it’s never worth getting the Elite upgrade and all the other upgrades, like Parthian Tactics.
Which gives me an idea for a new Imperial tech!
Creates a fully-upgraded Elite Unique Unit.
Costs 75% more than the standard unit.
Basically this would allow a player to get a few unique units for a specific purpose, at the cost of paying substantially more for them. I think it could allow you to see some of the less-built unique units, mixed into other compositions in smaller numbers.