Both French Knights in Age 2 and Mongol horseman fill the role of early harass in different ways. Let’s discuss design benefits of harassment builds in the context of speed, damage, durability, cost and other bonuses. But before we discuss the AoE4, let’s discuss the harass type roles of units in Starcraft 2 to compare.
Strengths: Fast, can heal outside of combat, aerial mobility, good damage in high numbers & easily massable.
Weaknesses: very fragile, weak in small numbers, somewhat expensive, easy to counter
Strengths: Can instantly teleport, very tanky, deals good damage over time and alone, aerial mobility, hits early forcing a response.
Weaknesses: very expensive, very slow moving
Notice how each have a good balance of strengths and weaknesses. BCs in the early 5 minute mark of the rush build, require a very specific response, and aren’t easily countered (at that time) in TvZ, because you’ll only have spores and queens. But the slow movement and heavy investment, make this not an overpowered harassment. Now let’s look at AoE4 harass builds in the context of French Knights and Mongol Cavs opener
Mongol dark age cav
Strengths: fast, hits early, good damage in high numbers & easily massable
Weaknesses: somewhat fragile, weak in small numbers
Strengths: fast, good damage alone, tanky, can heal outside combat, not easily countered
Weaknesses: somewhat expensive
As you may be able to tell from these comparisons, French Knights are a very strong option and dominate the meta currently.
My Suggested Change - Remove the chivalry upgrade from the knight. Give the mongol cavalry a healing outside of combat because they should be rewarded for being kept alive, given their fragile nature, they would fill a similar well-designed role of the Mutalisk light harasser in Starcraft 2. As it currently stands, the mongol cav opener isn’t amazing, and as time goes on people will only get better at defending against this. An alternate change would be to at least move the Chivalry upgrade from age 2 to 3 or 4, but in my opinion, a unit that tanky, that fast, that deals that much damage, has no business having a healing ability.