When people say some trading method or strategy will fail, what does it actually mean? If it means it will definitely fail, the difficulty level to know it for sure is the same as saying a method will succeed. Anything in between is just a random bet. Trying a ML (machine learning) method, curve fit or over-fit are common mistakes because it will mislead to a totally failed system, that is, a wrong design. Here the word "failed" is a totally diff thing in ML from what a trader thinks.