It should be noted that, the only Feudal UU we have right now, is really bad in the Feudal Age, and not at all worth producing over the M@A, even with the Donjon building meme strat, that almost always gets crushed.
If they add more Feudal UUs, they should stay this way, and be worse than base units, to the point that you would lose any competitive match because of them.
No one should want a Feudal Age unit that dominates as much as the Janissary does in the Castle Age.