The biggest problem is actually building it. A gun doesn't need legs. Wheels do the job just as well and are easier to maintain and with far fewer things that can go wrong. Get sand in even one joint and you have a problem with the whole thing. Also, the bigger it is, the greater the forces acting upon it, so the metal could buckle under the thing's own weight. Not to mention that it would basically just be a giant target, so if it can't move fast enough, it would be just a sitting duck.
In other words, modern technology can't overcome the drawbacks of such a design in order to make use of its advantages. As for those advantages, if it could be built, then it would be an incredibly versatile weapons' platform. A mecha would be able to traverse ground that ordinary vehicles would not. The fact that it has hands with fingers means that it can change weapons on the fly and even use specialised equipment, which again, could be swapped on the fly. Moreover, the hands themselves could be used as weapons or tools.
Long story short, yes, they could potentially do well in battle, but only if they can actually be built at sufficiently technologically advanced a level to actually be worth it.