Concerns about flushing cat waste down toilets derive from the existence of a parasite called Toxoplasma gondii.
According to the United States Centers for Disease Control and Prevention and the Mayo Clinic, risk factors for T. gondii infection in humans and other warm-blooded animals include consuming raw or undercooked meat, ingesting water, soil, vegetables or anything contaminated with oocysts shed in the feces of an infected animal, and drinking unpasteurized goat milk, and, yes, sewage.
With regard to cats spreading T. gondii, steps to prevent infection include disposing of cat litter daily, wearing gloves when gardening or working with soil, and washing hands promptly after disposing of cat litter.
In fact, it is recommended that all cats should be kept indoors and fed only food that has low to no risk of carrying oocysts, such as commercial cat food or well-cooked table food.
Cat waste found in soil and water that surround the homes of outdoor cats, and cat waste disposed of in landfills, produce harmful gases (CO2 and Methane - greenhouse gases that contribute to global warming), AND pollute the local environment, including the water and soil.
In closing, regardless of the manner in which you dispose of cat waste, the possibility of spreading T. gondii exists.
Although mild, flu-like symptoms occasionally occur during the first few weeks following exposure, infection with T. gondii produces no readily observable symptoms in healthy human adults. In infants, HIV/AIDS patients, and others with weakened immunity, infection may cause a serious and occasionally fatal illness, toxoplasmosis.