Relic had to really scratch their heads to make Factions different in
COH2. And honestly, people did NOT like that.
Solution? Make COH3 w USF-reskinned brits. Ta-daa well balanced brits 
Now we can have Japanese Tigers in Italy that don't break the balance!
The time for mirror stat factions has arrived! As I predicted.
I think it's just that people don't like losing. Most of the balance discussion on these forums comes from people who prefer one faction and play it more frequently than the others, and when they lose they blame the game before they blame themselves. That isn't to say that the game hasn't had its obvious overperformers (vanilla obers, lol) but every iteration has had the same people coming to the forums complaining about the same factions and grasping at straws to make an argument for why x unit was/is OP. Relic simply needed to refine the game with slight tweaks to unit costs and stats, but instead felt the need to redesign the game numerous times and here we are now.
That being said I guarantee you that if we copy paste axis and allied factions so that they're mirrored and they only thing that distinguishes them is the skins, the same people who come on here now and bitch about balance are going to be complaining about which army gets the coolest color scheme and how that makes them have an unfair advantage.
If CoH3 is to avoid the blunders of CoH2 it's going to require a rigorous inhouse testing and feedback phase (maybe include some of the higher ranking players in both CoH1 and CoH2), limit the amount of "Army A is strong with early game so MUST be weak in later game" thinking that goes into faction design, and design every faction so they come with and can easily access the basic tools (snares, at and ap mines, mgs, indirect etc.). As nice as it sounded, community driven balance patches really are the wrong way to go when it comes to the longevity and overall health of the game - Concurrent player counts are indicative of this and show a correlation between "balance patch" go-live dates and player count dips. I hope Relic has learned it's lesson and is more careful with community influence in the future.