It's cheaper to keep producing units than upgrading them for Comintern. You will the advantage of superior numbers but each stack individually will be eaten away faster by late game stacks. This doesn't hurt you that much as you can replace your units fast but it isn't good for our Kill/Death ratio. If you don't upgrade you can produce units close to the front in level 1 production buildings. But the further you go from your cores the less effective you will become due to longer supply lines. The further away you are from the front the more you should upgrade your units instead.

For smaller maps like the 10, 12 and 22 player maps I would upgrade only stealth and scout units for the obvious reasons of staying stealth or be able to continue to scout.

On larger 30, 50, 100 player maps I would spam a lot of level 1 units and then start upgrading them later on.

Watch this Comintern doctrine video at 13:06 "to upgrade or not to upgrade?"

Call of War | Comintern doctrine explanation, best units and strategies - YouTube