I've noticed that ever since the "Together We Rule" Expansion Pack, the AI REALLY likes to build stealth units over regular units. Not only does this over inflate their combat strength, but it also cheapens the warfare of the battles, as they rush towards Musketeers and Line Infantry, being one-shoted in the process. I would suggest to slightly raise the cost of building it to make it less likely for the AI to make the unit, or just add a variable to reduce the chances the AI makes the unit. (Possibly removing it if the AI's culture is Dipomatic and/or has a Unique Unit Replacement)