Eagles would be easy to rebalance because there are only 3 civilisations that can build them.
If Inca and Aztecs don’t get the technology (Aztecs already have strong infantry) and Mayas unique technology gets imply changed to adding 30HP instead of 40HP.
Unique units would be less of an issue too. Just remove 10HP from every unique unit of every civilisation that gets access to the technology.
But that could also be the change to buff a few less powerful unique Infantry units.
It would potentially change the power spikes of those units and make the upfront investment higher.
Depends on what age the technology would be at.
If it’s an Imperial Age technology the stats of the none Elite Unique Infantry units would not have to be changed for example but most Elites only have +10HP compared to the standard version so they would all need to be changed to +0.
I still think all infantry should be affected by this technology.