I don't think legs will be the big weak spot of a humanoid mech, it'll be armor. These mechs are huge, covering all that area with decent armor would weight WAAAAAAAY too much to keep the thing mobile so you'd be stuck with armored car levels of armor, can't even slope it properly (armor angled more than 45° from the path of the projectile deflects some of the force outright and is more effective). A frigging Browning M2 would rip through that. The whole point of a tank is to withstand MG fire so it can move through areas that would be kill zones for infantry. Gundams are on the level of Nazi supertanks (P-1000, for example) of impracticality, there's only so much weight that your engine can move and the more weight that is the more likely it'll break down.
Modern tanks are built extra flat so they can take cover more easily, hiding the more vulnerable (relatively speaking, turrets have like 50% thicker armor) body of your tank behind a hill and only having the turret poke out makes it much harder to spot and kill. Making your system stand vertically is the exact opposite of that.
Also the biggest threat to tanks are urban combat (where infantry can move unseen into the weak areas of a tank and deliver an AT rocket at close range, on an open battlefield the short range of those things and inability to deal with modern MBT front armor makes them significantly less effective) and air strikes (especially helicopters). None of these benefit from a bipedal mech either.
If you want a legged tank then build something like a crab with many legs that can be replaced on the field and stays close to the ground. Plus if it can stay mobile with a few legs destroyed it's more durable than a tank when it comes to hits to the movement system, a tank has only two treads and if one breaks the mobility is heavily impacted. If a HEAT or AP hit to a leg could only destroy that leg and have no real impact on the main body you'd have highly increased durability to side attacks.
Xain-Russell said:
As much as I love giant robots, I have to agree with that one guy who said they should be building robots to help clean up after a nuclear disaster. It's not that I think a mech would be impractical (I do) but I'd rather the nations of the world build machines to better our lives, not to destroy them.
That's easier said than done because radiation damages circuits and corrupts memory. Imagine a constant EMP going off. NASA uses really old hardware with huge wires and transistors that require a lot of energy to flip because of the radiation in space.