Whether they're great or not is not important; the fact of the matter is as follows:
1) They largely only produce light tanks (WW2 -> now) as their main battle tanks. The French AMX series is a very interesting tank, as it is a tank with a 20mm autocannon (good against infantry).
2) The
stereotype is that they flee really easily
3) In World War 2, they had a really strong air force. They only used them for recon', though, following old World War 1 plane tactics.
4) They make nice helicopters
Whether Napoleon kicked everyone's ass or not is irrelevant, what pertains to this game mostly is modern history. So, militarily, improved light tanks and aircraft is ideal. It'd be comical to also make their units rout, and, honestly, it'd be a pretty cool game feature if units routed regardless of faction. Maybe make the routing threshhold lower for France (like it is in Wargame: European Escalation).
A good unique unit for France, in my opinion, would be a medium tank with slightly less health, slightly more speed, and a 20mm autocannon along with it's medium tank gun (secondary weapon).
http://en.wikipedia.org/wiki/AMX-30
Or air power +10% as their helicopters are used around the world.