You can't define balance in this situation. There is just skill difference.
IMO there are lots of things that can be balanced seperately that have nothing to do with skill. Among which are emplacements which suck in 1v1s but can be a great pain in the arse in team games. Bunker spam falls under the same catagory. Or faction tech and unit prices.
Then there's stuff like Panthers or Jacksons having to be good on their own for their cost in 1v1s because one would never get two or more there, but then becoming borderline OP in team games where they can be used en masse. Infantry Sections that are bad in the mobile nature of 1v1s but excell in the tight defensive spaces of corridor team maps, crutch light vehicles that have to be toned down in 1v1s to prevent them from running rampent across the entire map while in team games they can't manouvre and run into AT guns at every corner, etcetera.
Not to mention faction mechanics that can synergise too well, such as the USF ambulance or the Soviet FHQ, or commander abilities in general.
In the end the vast majority of games will probably be played by solo people or friends queuing up for random team games, so it makes sense to balance the game around these. The advantage of having seperate balance (or even an Esports mode) is that the game can still have a competitive scene while also being balanced and fun for the average player.