When looking up items concerned with health risks the first thing I look for is who wrote the article. Was it a medical professional or a layman? If not a medical professional do they provide links to substantiate their claims? If not then what is written could be and often is erroneous.

I can think of several potential health risks with fisting.

Hemorrhoids would be a major concern when dealing with anal fisting as even having large bowel movements over time can cause this problem.

The wall between the vagina and rectum is thin and can be ruptured.

Urinary incontinence is another potential risk. Even though the vagina is made to stretch for childbirth many women can attest that after multiple pregnancies the muscles just aren't the same. Stress incontinence (dribbling when sneezing or coughing) is common in women who have multiple childbirths. Kegel exercises are used to help this problem. A loss of muscle strength due to over stretching in one area of the groin can lead to problems in another area.

Infection from tears (even tiny tears that can't be seen) would be a huge concern.

These are a few of the things off the top of my head.

As a medical professional I would discourage fisting because of potential health risks. Those who do practice this should take whatever precautions needed to decrease the risks to their health (lubrication, careful cleaning of the area with the appropriate cleansing solutions, going slow, etc)