Nothing is wrong with fermented soy products, such as tempeh, miso and fermented soy sauce. In fact, fermented soy products are good for you and many of the proclaimed benefits of soy are attributed to fermented soy and not, as some people would have you believe, in the more regularly consumed unfermented items such as soy milk, tofu and edamame.
The people that believe unfermented soy products negatively impact your health cite hormonal disruption, nutrient absorption issues, thyroid impact, fertility issues, toxicity, etc. as potential issues, but there are some who argue that the science behind the blaspheming of soy is not exactly compelling.
In my humble opinion, there is enough research out there that makes me feel that unfermented soy is a bad idea (especially this 1999 letter from two FDA soy experts, wherein they protest the FDA approval of soy), so I avoid soy milk and tofu like the plague and limit consumption of other forms as much as is realistic (it’s hard to avoid completely – check your ingredients and see). However, I do think fermented soy products are very beneficial and worth eating.