I thought some folks would throw more specific comments in about the benefit in relation to cost for the units. For example, in a slug-fest between two opposing factories, I think Artillery combined with about two Infantry each will make a good tough core force. I would expect to see a group of armor mingled with the core fighting groups, and a group of TAC bombers to match. At 7 IPCs, a pair of Infantry/Artillery deliver a good punch. It would be nice to have a couple extra infantry to absorb defense blows to protect those artillery, so I figure 2 Inf per Arty is a good starting point.
In the earliest stages of the game, mech infantry and armor buys may be key to get some support onto the front lines when there is a piece of key terrain you want to get early. After that, unless your supply lines get too long, I suspect to see few mech infantry buys and only a few armor buys to match TAC bombers.
Surprisingly, I expect to see Japan build a factory in Manchuria, followed buy many mech infantry and armor buys to chase Russia and pick up some IPCs from their wide open spaces. I also expect this to change the way the war is fought in China, but that is another subject.
I hope to have the time to publish a numeric analysis of the units here. I would hope folks would respect that the analysis is strictly an academic what-if and not a defenitive “buy strategy.” It should guide some folks to keep them from getting too far off the reservation though.