Finance Guru is easier because there is already a societal expectation that some investments will take decades to pay off. I could make a program about Whole Life Insurance or Mutual Funds and caveat the results with the phrase "you need to keep investing over the course of 20 years, market volatility can have an impact on returns" and that'll cover the people that are disgruntled.
With Fitness, no one has the expectation that it'll take years. If you're going to a fitness guru and you don't see results in six months to a year then you know it's a sham.