I give the game 9 outta 10!
Mix amphibious units with true naval units and some underground Earthquake generators and you have something they can't stop no matter what! Too bad it only shines on them naval-style maps... Anyways, it isn't too difficult, for the most part anyways... I suppose true difficulty ratings depend on your faction of choice. The Lunar Corporation (LC) is the easiest, though a pain with them solar panels n such. The Eurasian Dynasty (ED) would be the middle difficulty (to me), good power supply, lots of cannons and such. (kinda ammo intensive though) The United Civilized States (UCS) is a pain! If you play these fools, build no more than 3 Harvesters per Refinery (2 land, 1 air or vice versa) or be prepared to wait for that money longer than you expected. Their power grid is easy enough to handle, and them Fortresses are excellent base defense. I held off and entire ED assault force (ground and air) of about 2000 units or so, with 3 of them. :) 2 Heavy Plasma Cannons and 2 AA Plasma Cannons on each fort makes for a tough defense line.