Well, from an artistic and anatomical standpoint, I would say he is pretty well proportioned, including the arms and legs. The probability is very high that they have taken the strength of the joints into account for the arm cannon, and, ratcheted joints should be able to support it. And, given that the cannon/scope will likely be made of plastic, I am pretty certain it will be light enough for the arm to support it. As Piranacon pointed out, you will never get a completely cartoon accurate model and still have it transform. Larger arms, and thicker legs would ruin the gun mode.
As for why I prefer the appearance of the alt mode to be spot on, that is simple. The alt mode is the robot’s disguise. If it isn’t convincing, it isn’t a very good disguise now, is it? This model does a very good job of making a convincing Megatron without sacrificing the alt mode. He is well proportioned, fully articulated, nicely detailed, and much closer to the cartoon version than he is to the original toy version. Is he spot on? No, but that is impossible to accomplish and still have him transform, and for the reasons Piranacon mentioned. There really isn’t much else they can improve on him.