You've gotta think about how and where a Mech would and could be utilized. First of all, a Mech, as we know them, has very few real world advantages over tanks, helicopters, jets, etc (cause all to often, mechs have to fly in anime/cartoons).
Metal Gear Solid (REX) was actually a pretty decent idea; bipedal nuke launching platform. Nuke launching platforms were a hot topic during the Cold War, and the idea of a platform THAT mobile launching a nuke from any terrain would have aged Kennedy 10 years older in a heartbeat. That's a perk for mechs.
The negative is tactical placement. Lets assume the whole top heavy challenge is a non-issue. Where would you place this mech? To stop anti-tank shells, it'd need to be armored. It's supports (legs) would need to be powerful enough to maneuver with all that armor too. So we're talking several TONS! Not tons of weight spread out over 4 wheels, or two treads. Several tons Coming down on two points. Lets explore that weight issue;
- Most urban enviroments are hollow. The streets have things running under them.
At best, treading on cement would be like a person walking on packed snow. The risk of tripping would be great. I doubt the dexterity to catch ones fall in a mech is as good as a persons.

Worst case scenario, the mech just collapses into the sewer/subway/underground network in whatever urban environment.
- It'd sink itself in soft dirt, mire itself in mud, trip itself on trees it knocked over itself, slip on rocks (boulders), or cease functioning in water. A building couldn't take them. Not on one floor. It'd crash through the roof (helo drop/plant/careful placement), tearing through floor after floor.
Terrain is an issue. But flight solves all terrain issues... right? WRONG! We've established that this is a heavy monster. A mech would be. So to make it fly, we'd need to tie in several powerful motors. Excellent! Now your Mech can fly. So how is this bad? Lets explore that;
- A flying mech is a flying target! Bigger than a helicopter, less manuverable than a jet, it's the perfect target. To survive an onslaught of any aircraft flak, it'd need to be more armored than anything, putting cost to fly unreasonably off the charts (weight to power-out put ratio). Even then, the flight system would probably break first, and the fall would kill the pilot. So aerial manuvers is an issue.
Well, we could put a mech in SPACE! No terrain to worry about, jets and choppers can't compete. What's to stop a mech from dominating in SPACE! Lets explore.
- A mechs function as a mech is a weapons platform and armor, much like a tank. It's so called advantage is its two legs. But once you put a mech in space, those legs become useless dangly bits. Excess mass to get in the way of propelling its already impressive mass through the final frontier. Why have legs on something that won't ever stand on a surface?
- Unless... the MOON! Or similar
low gravity no atmosphere environment! A walking walk (mech) could be quite the military advantage there. +1 benifiet.
I believe the tally so far is +2 good, -4 bad, for a total of -2. IS there any more? OH hell yes there is. But the biggest one I can think of so far; COST.
To make that marvel of technology would be the most costliest endeavor of military vehical/personnel equipment since the last war. In a world were military equipment is crafted by the lowest bidder, a mech would seen to costly to field. If it ever broke, the military would go bankrupt maintaining. (I'm exaggerating with that last sentence. I imagine America would just sacrafice our educational system fund to see repairs exacted.)
So, in the end, a mech could be useful in low G off world military exploration of the cosmos. But here on earth? Notta chance.