I don't care for the plastic 66 bobbins either. I don't think I even have any of the plastic 15 bobbins here. I think they rattle, and I always thought they might be "different" as far as tension because of their weight. I've also never cut myself with a metal bobbin.
It's a shame if the metal ones are worse. I was always "taught" metal bobbins for metal cases, and plastic for plastic bobbin cases. Maybe I'll order some more of the ones from the supplier and test them out again. I had no concerns about the quality of the ones that arrived that first time. Good fit, fit on the bobbin winder, proper notch, etc. I've seen some of the cheap cheap metal 15 bobbins that barely have a notch, so I know what you refer to.