I have used metal bobbins for years in my mechanical, electronic, and computerized machines (I have Kenmore, Singer, Brother, Janome, and Euro Pro) with no problem but I learned something new today!
I have machines that have front loading bobbins and top loading bobbins. I took my Kenmore Elite in today, which has a top loading bobbin, for a recheck by the technician. I noticed he changed my metal bobbin out for a plastic one while he was working on it and I asked him why.
He said the sides of the bobbin cases in top loading bobbins are made out of plastic and the metal wears the plastic down. Also, if there are any knicks in the metal, it could cause the thread to gritch so he prefers the plastic because he knows there won't be problem.
He said the bobbin cases in front loading bobbins are made completely of metal so a metal bobbin always works great and won't have a problem.
I told him I had used metal bobbins in all my machines for years with no problem and he wasn't surprised but he did point out that I would probably have to replace the top loading bobbin case in my machine because eventually it would wear out. So I'm switching to plastic for my top loading bobbins.
Oh, my machine was stitching funky. What I thought was a tension timing issue was actually a worn out presser foot. Well, it was plastic and the machine is nearly 25 years old!