Why is it a terrible idea? Because organic things aren't good at fighting. Humans were built with endurance in mind. We're incredibly resilient. If we suffer scratches or nicks, we can heal them up in a few days. Deeper cuts can be healed and replaced with tough scar tissue after a few weeks. Broken bones? No problem - those can be healed as well. Internal bruising, fractures, and sprains? All can be healed up. The point is that we're designed to be able to recover from repeated minor to major injuries over the course of time. As long as nothing gets detached, and the we aren't killed, of course.
That comes at a cost, namely short term, something which humans are remarkably bad at, compared to other animals and things like robots. Steel limbs sound great, but they can't be remade if broken without invasive surgery. Similarly, armor can block blows much better than skin can, but can't regrow from nicks and cuts. And it's weight means shorter operating time. So, short term - having solid but unrepairable defense is better, but humans were built long term in mind.
And combat is short term. Very short term - a one on one fight to the death can take seconds, especially if side A is built for short term victory and side B is built for long term endurance. Humans aren't built to fight life and death situations - they're built to avoid them or use their big human brain and dexterous hands to manipulate the situation and create tools to give them an advantage - tools that give them short term advantages, like weapons, armor, and tools.
In other words, humans aren't built for combat, and anything designed solely based on the human physique is just subpar for combat because the same materials could be used on better designs. That aside, you run into issues like the square-cube law, meaning that building humans to scale for double or triple their own size is almost impossible because it needs materials with a strength that scales exponentially.