its really only new GMF's for robots... since stats must follow a set formula
Even that must be regulated. Example: I could make a huge box that totally encompasses my bot so the chassis can't be touched, and also does damage. Or I could make a reeeeeaaaallllllly long blade that covers the whole arena.
I think the best way to ensure balance is to make a list of allowed custom components, instead of letting people make their own. Like you could take a subset of Firebeetle's components, Dummy's components, and Lu-tze's components and say "you can only use these". Then, even if some of said components are unbalanced, it's fair because everyone has access to them.