This image shows a cross section of the long and complex evolution of the combat robot from it's massive and clumsy prototypes through the cheap mass produced units of GR: 1999 onward. While the superior durability and strength of earlier designs means that they are still sometimes deployed by various armies in most cases they have been surpassed by units developed in the opposite direction for superior combat strength and weapons load out.
As you can see the earliest combat robots were not even capable of using ranged weapons, being restricted to massive axes and other crude forms of dealing damage.
The next major iteration boasted several improvements, including a slimmer profile, and integral ranged weapons. However this design had a key weak point at it's waist, which was solved in the third gen robots.
These robots, sometimes called Androids, are effectively a minifig, but boasting on-board weaponry. Considered by many to be the pinnacle of combat robot design they lack the torso pivot of earlier models, but are cheaper to produce and field than the older robot types.
Finally we come to the crude shoddy devices of the GR: 1999 trade wars and onward. Based on the assumption that robots were intended to be disposable troops they once again have the serious vulnerability at the hip joints and no longer even sport integral weaponry. Some whisper that this design was in fact stolen from engineers in the Negaverse and that each of these robots has a tiny shard of CLOan ABS as it's power source, which has led the LegitEye Knights to attack formations of these simplistic mechanical troops more than once.