There are a couple of different types of flushes. The two most common are powder and liquid. They both are effective and can clean your radiator, but they both pose different problems.
Powder flushes have more inherent health risks. You can accidentally breathe it in or it spreads to the skin and it causes chemical burns. Powder flushes are powerful but have more risks associated with their use. Liquid flushes are also caustic, and you will need to take the proper precautions to guard against spills and splashes.