Comintern Doctrine

You also forgot that the Comintern doctrine has tons of buffs unlike he allied and axis some have huge bonuses so to the point you can have cheap and also some strong units like sp rocket artillery which is better than all of the other doctrines but at the same time is also strong and a lot cheaper.
At the end game many countries will have to deal with a ton of units and their upkeep but Comintern is basically unfazed they can go past any other troop in the game and not care about upkeep and very overpowered in early game

Early game it’s still weaker damage wise and will lost a fight with another country, especially axis. The trick is to not get into a major fight (unless you have an advantage) and focus on making an economy. Then with a good economy you can make a high level and large army, and push into other countries snowballing. In the late gam as you said upkeep is not as bad and you can make a horde, surpassing all other doctrines and being able to overwhelm them.

Com Intern requires a slightly different mindset which doesn't sit well with a lot of people.You have to be prepared to lose troops and it will effect your k/d ratio.
Building economy takes time to pay back, economy by expansion is more effective at the cost of army health. . .
Cheap units isn't worth it if your opponents have stronger economies. . .

Expanding with com intern on the first day is about pumping out arty and inf, then when you get RA up and running things get a bit easier. If you can take an AI or Comintern player early that's a good start.
Taking on Axis is problematic due to their strength.
Allies is easier but they can rebuild quickly so you have to press your early advantage and clean up quickly.
Pan Asia depends on if the opponent uses terrain effectively, generally taking Pan asia cities is costly & is like Axis.

Early expansion is hardest for Com intern & Allies until you get RA and Tac bombers respectively.
Axis uses it's initial army strength to bludgen neighbours. Pan Asia has lots of early unit buffs and speed.

I play a lot with a team-mate that prefers com intern. I find overall that:

I can match unit numbers as Pan asia by expanding faster & having a stronger economy. I'd have stronger army strength.
I can match army strength as Axis but less units, most units will have damage & need management.
With Allies I'd have fewer units, but once I make tech jumps the advantages in Air power due to increasing range are the most significant.

I'd put Com-intern on par with Allies.
Axis is dangerous at the start but hard work long term, If you can Get RRG production going, the extra range can be a game winner.
Pan Asia is always strong. Getting RRG up and running actually beats Axis RRG's if you know what your doing.