Wow! Fantastic piece of work. I know about the weight problems, the only mech I've ever built had to have the joints locked in position. Otherwise, he would crumble the second I stopped supporting him with my hand. The head of your mech looks great and I love the hands. Very well done.
Quoting Green Ghost .
You should try to get more movement in the shoulders.
They're worm geared for full forward and backward rotation, and they can swing out. Due to the overall weight of the model, not much else poses well, so it looks a little awkward to just have the arms doing stuff :-)