MECH is quite expansive, but that's mostly due to the fact that they are also incredibly powerful.
A fully mechanized tank division becomes extremely fearsome due to the hardness increase, to the point where even INF with enough pierce won't stand much of chance against them.
Also don't forget that MOT gets a significant hardness boost from unlocking MECH I, so you have sort of an abstracted way to model the advancement of the early mechanized and semi-mechanized equipment.
A fully mechanized tank division becomes extremely fearsome due to the hardness increase, to the point where even INF with enough pierce won't stand much of chance against them.
Also don't forget that MOT gets a significant hardness boost from unlocking MECH I, so you have sort of an abstracted way to model the advancement of the early mechanized and semi-mechanized equipment.