I see it differently than a lot of you. No spearman or medieval unit should be beating mech inf. because 1)a machine gun, properly supplied with ammo, will mow down hundreds of people before being overrun if the other side has no firearms and 2)the weapons of spearmen and med inf., a metal tipped spear and a ball and chain respectively, would have absolutely no effect against the reinforced armor of a mech inf. You couldn't even get it through the window of one of these vehicles. And as for the guerilla- it's feasible that he may eventually be overrun, but med inf. have to be at point blank range and I assume any half decent shot should be able to kill a guy moving slow in heavy metal armor with an AK before he got to him.
For those reasons, I'm really in favor of adjusting the combat scaling so that it is much more unlikely for units from a lower era to effectively compete with those above.
Hey, it's why horses weren't much used after WWI- they died too easily and motorized transport and weaponry basically made them obsolete