It has UL181B approval, but I wouldn't use cloth backed duct tape for anything related to air ducts.
Interesting study that was done on a variety of UL181B-FX rated tapes. It is primarily testing performance on flex duct, but they did a small test on metal too.
The bits that are pertinent to your question is on page 11, and page 13.
The cloth backed duct tape, which is what the Nashua 557 tape is, was the only one to fail completely...