Look for the *notice* to skip ahead of all the introductory and evaluation and get straight to the point because I know people won't read it all and will just call me a Treaty noob.
Not many other units have had the controversy of the Cuirassier. Maybe only Abus Guns or Longbowmen have been brought up as many times. And as many times as it has been brought up, the unit remains unchanged.
Some people think this unit is OP, but with it's price tag, is too expensive to be a balance concern. Some think that this unit needs a small nerf late game when it is easily spammed. Some think the Cuirassier is actually inferior to Veteran Hussars. Some feel Thoroughbreds is the only real problem. And we all know what Treaty players think. But I think in small numbers the Cuirassier is fine and needs no change, but with some significant numbers, Cuirassiers are a little too strong.
I have seen many balance ideas floating around on how exactly to fix this unit late-game without screwing it over early game. I will list what seems to be the most popular ideas.
-Give Cuirassiers Colonial stats
While this would help with the insane late-game stats of this unit due to multiple upgrades, I don't think the stats are the underlying problem. Lancers and Halberdiers both have veteran stats, yet no one complains about them.
-Nerf Cuirassier Imperial upgrade
I don't think ES would want to do this because it simply changes the design and layout of the game. I know I wouldn't want to pay 3000 resources for a unit upgrade that didn't upgrade by 50% when all my other units did.
-Add a build limit
This one is just a silly implementation. I know Spanish players would be mad if their lancer was capped and dutch players would be mad if a build limit was put on their halbs. While this would work, I don't think this is the best answer.
-Add .5/75 negatives vs HI/LC
This is the 2nd best idea I've seen so far on forums only behind colonial stats. But IMO, this idea unnecessarily nerfs Veteran Cuirs, which are fine in the numbers one normally sees them in Fortress.
Out of all these suggestions, I don't feel any of them address the underlying issue of Cuirs. And that underlying issue to me is splash damage overlapping when several Cuirs are present.

Thanks to AI Guy for his work here at HG.
Look at this picture, notice how the Cuir damages 5 units. Now imagine 2 more Cuirs above and below him. Now you have 3 Cuirs all doing splash damage overlapping each other's splash damage. But 3 hussars for example can and will only attack 3 units at once until killing that unit and moving to the next one. While Cuirs attack all at once. Using 3 Cuirs in that situation would almost be like using 9 smaller weaker hand Cavalry.
*notice*
And as I was looking for a suggestion for this problem, I told myself that what ever change I made would have to make sure that Fortress age Cuirassiers were still viable to use and only the late-game mass of Cuirassiers was effected. I began to look at the damage cap of Cuirs to see how that would work.
If you don't know, the damage cap, according to the above link, is supposed to help control the damage output due to area of effect. There is more info in that link. A Cuirassiers base attack is 30 with an area of 2 and a damage cap of 60. According to the studies, a Veteran Cuirassier should be able to inflict a maximum of 90 damage to units with base damage and the cap. It is also stated that the cap is upgraded whenever base damage is upgraded and that most caps are 2X the damage of the base. So my suggestion is to give Cuirs a damage cap of 45.
I ran a series of tests to determine and gauge the effectiveness of Cuirs. Note that I did use attack-move for the Hussars and Cuirs to try and reduce path finding from being a factor in the outcome.
I started out with 80 Veteran Musketeers(8,000 Resources) on team 2 and 60 Veteran Hussars on team 1. 80 Musketeers beat 60 Veteran Hussars(12,000 Resources) easy enough, not bad for a 4000 resource difference and muskets came out on top.
Next I tried 40 Veteran Cuirassiers(12,000 Resources) vs 80 Veteran Musketeers (8000 Resources) with the default damage cap of 60 for Cuirs. Cuirs won with 22 Cuirs left over. I expected this. The average over several trials was 21-22 left over.
Now I tried the test again with the same numbers but this time with a damage cap of 45. Cuirs still won but 9 Cuirassiers left over. Average was 9-10 Cuirs left over.
I think this is a significant improvement over the 60 damage cap. These trials show that Cuirs will still be better than Hussars vs HI and most other units, but have the issue of splash damage overlapping reduced when large numbers of Cuirs come into play.
I'm open to suggestions, comments and opinions. If you have an opinion please back it up with reason. And please no flaming.