1. The Cuirasser can not be effectively countered by its hard counter units, halbs and dragoons (goons can but it takes serious microing). Massed cuirassers have become almost impossible to stop between two equally skilled players.
SOLUTION: Give halbs and dragoons strong armor against trample damage (note this is not a nerf, but an improvement to all civs units). It will keep the Cuirasser powerful and will make it worth the 300 resources it costs...but it will allow the units that are supposed to counter them to actually do so.
2. The Couerer de Bois is too powerful and the French economy is too strong and the extra attack/hp make them too hard to raid effectively.
SOLUTION: Oh wait, that is their civs unique bonus. There is no problem here. You are just going to have to learn how to raid or rush better to keep their economy manageable. If something absolutely had to be done...add 5 seconds to training time MAYBE. I don't even think this is necessary thoughthough.
3. The early skimisher card. A lot of people have a problem with the early skirmishers being too powerful.
SOLUTION: Build some Cav!