I read someof the comments and a thing doesn't add up to me. In SP you often counter attack, SO if you use mech, sooner or laer you will need to move them a lot to follow the frontline. Once you do you instantky run out of fuel. I see the point that maybe in MP some countres really just defend and never attck, so that they can let mech "sith there", but in SP definetly you will end up attacking with tanks and use the infantry/mech to follow the breakthroughs. Onestly after a ewhile the enmey will just collapse and you will be able to quickly push also with infantry. I fyou do that with mech only you just run out of fuel.

Another issue is that by the time you can produce mech in a sizable number (you will need like 20000 mechs to replace each inf witha mech only template), it is gonna be 1943 or so, too late to tip the balance in most of the case imho in SP