All the issues faced in 3's and 4's are just the issues faced in 1's and 2's but cranked up to 11 due to the added resources and unit counts.
Allied early game gets stronger, Axis late game gets stronger. If the factions were designed around being good at all points in the game then this wouldn't be an issue.
Teching needs to be standardized across all factions so certain ones don't get to their late game units faster than others, call ins need to be tied to teching. Unit power should be a logical slope upwards, not a fluctuating mess.
USF and OKW's teching is non-linear and makes no sense power escalation wise. Your going from basic units to your late game units in literally one tier.
Soviets and Ostheers teching makes more sense, but Soviets teching is still bonkers because you only ever build t1 or 2, and then on to t3 or t4 so again your going from you very first units to your very last late game units. Ostheer of all the factions is the only one that make sense with a logical progression upwards in the power of it's units
All the factions should have the same logical creep upwards in power as Ostheer, not this insane stuff we have now were you get access to your best units at the start of the game or right in the mid game leaving little point to go backwards, at least OKW gets some incentive to do this.
Once all the factions get access to their units in a more sensible fashion the issues with team games will by and far go away because everyone will be as strong as the other team at all points in the game as the game progresses.
What about call ins? They should be tied to teching.
The lack of Allied unit diversity is a key issue. I think it will be addressed in the future though.
Soviets don't lack for unit diversity at all, Ostheer is mostly just gren spam and USF has literally nothing else they can make. OKW has a lot of diversity in mainline infantry but none in any other area.