I wouldn't say it's not good. I would say that usually SE would be better. Depending on how you sharpen you're PE edge you can definitely get good performance. SE will probably cut longer though. I've cut many nylon straps and usually the SE performs better. But if you're just cutting 1 seat belt it doesn't matter much. I think blade shape would make a bigger difference if both edges were sharp. Hawkbill or sheepsfoot would do better than something with a lot of belly. Even a recurve would do better than say a shorter drop point, clip point, leaf shap, etc, etc.