Factories develop too slowly to be optimal. Especially if you are siting them in formerly enemy territory (G2 take G3 build G4 first units G5 first attacks??). that side of the board is covered in factories already.
Mech Infantry are really weak units when viewed in isolation–weak attack, outrunning the artillery, don’t combo with tacs, expensive.
But on the massive Global board, they are awesome, and it is infantry that is weak. infantry’s only remaining advantage is that it is easier to load on a transport, so you’ll always need that. But for the $4, the mech is tossing pairs with artillery where you need it, rushing to the defense, or best; stacking for the final assault in front of your tanks and airforce. This last job is where they shine, and every early game dollar that you planned to spend on bases, factories, or slow infantry or artillery would be better as mech infantry. They can be on the front line as fast or faster than built units; once the mechs arrive; build artillery in Russia–use those productions slots to pair up. Instead of bulding so many tanks, more tanks survive. Similar Japan, though that will require factories for sure.