Flamers require no chemicals, and don't result in groundwater contamination or chemical residues on garden crops. But safety concerns of another type remain. Never use flame torches around any dry, brown, or otherwise flammable material. Also, their use during dry periods in forested or arid regions is prohibited, or ought to be. Always check with your local fire department or town clerk before investing in a flamer.
Personal safety is another issue.
These portable torches use pressurized tanks of propane and, if handled carelessly, can be hazardous. When you pass the flame over and around weeds, it quickly boils the water in the plants' cells, causing them to burst. Once the heat destroys any section of a weed's stem, for instance, water and nutrients cannot reach the leaves, and the top part of the weed dies. The weed may not droop immediately but will wilt and die within a few hours. Then you just leave the weed to compost naturally.