I warn you, I'll probably be quite critical about the design, as I want to look at the detail and I make a habit of analysing potential internal structure.
As a remake of the old ACU, generally it would suit the task quite well - it does retain a little of the old design feature, and the general aesthetics give it an authentic feel.
The Gatling-gun-esque weaponry mounted on the shoulders looks fun to use, but the hard-point of the weaponry seems spindly, with very little actual protecting it. In my experience the UEF trade manoeuvrability for lots of firepower and resilience, I imagine the faction would have adopted a more armoured approach to the anti-air units.
That's not to say i don't like them. It sort of reminds me of the T3 AG turrets the UEF used in Forged Alliance, a piece of weaponry that was always fun to employ, just because of the satisfying sound it made.
Likewise the arm-mounted weaponry and tooling suits the ACU well, merging with the overall design. I'm not sure if you intended it to be this way, but it seems that the bulkier, stocky weapon mounted on the left arm would be best suited as the gun, making for a short/medium range plasma-bolt type weapon, while the right-arm mounted item would be suited better to the task of construction.
The design critique which draws me this way is in the fact that for the ACU of it's size and stature, the right arm which I think you intended to be the gun just seems a bit small for its size. The barrel could probably fit a tank or two inside, but compared with the rest of the model the actual barrel of the weapon seems...diminished. In this case, I think the AC1 ACU had a better gun design, personally.
The rest of the design, specifically the backpack and vents, seem appropriate as well. It gives the whole mecha the feel of an armoured soldier, as if it would be your classic six-pack marine, another feel which fits with the ACU mindset and style.
Well done. It doesn't feel too big and bulky. It appears that it would function superbly. I do agree though, the structure will play a key role. How well do the joints function? Where is it's center of gravity? Do the parts phase through each other when you run an all around joint sequence? All possible joint movements should be examined.
I've sent a review of my opinion of the mecha, so here I just want to ask about the plans for AC3. I may be asking again when I've already received an answer, and apologies if this is the case, but do we have any ideas when we'll be starting? (So I can get an idea of what my future is going to entail generally for me.)