On one side you have nids and orks and the other you have mech eldar and land raider spam.
take that back!
orks can go mech too, and are pretty much the only mech formation with a vehicle that carries 12 men and a huge kannon!
you have trukks too, looted wagons (even squiggoths)
oh, and you can make a decent battlewagon and still be much cheaper than a land raider