According to the MSDS for Flex Seal, it contains: Petroleum Asphalt, Crumb Rubber, Petroleum Distillates, Hydrogen Sulfide and Other Proprietary component with no exposure limits.
http://www.mcoe.us/view/1668.pdf
According to the webpage for Flex Seal, the tape version is a "modification of the liquid" so the materials are likely to be the same with a slightly different ratio of ingredients so it's workable as a tape rather than a liquid.
Since it contains Petroleum Asphalt and Petroleum Distillates, I wouldn't apply it to my EPDM or TPO membrane, YMMV.