I'd split the nations into 4 tiers:
1. British, Korean, German
2. French
3. Roman, Chinese
4. Greek, Japanese
Seems to me British is as powerful as ever since the tactics change, decoys are very well capable of keeping mortars away from rifles. Germans are crazy with the fury, Koreans are crazy with 40% more rifle dmg + extra tactic. French is good but not as good as those 3, pure combat power wise.
Romans and Chinese have at least some offensive benefit, but Roman's troop space is not enough to be the equal of French's alliance troop space + tanks, and Chinese unique rifles + merc slot isn't either, mainly because the Chinese rifles aren't as good as British or Korean rifles.
Greek and Japanese should be avoided, pretty much.