I bought my first Brother last summer. I asked if I could use my metal bobbins and I was told if I used the metal ones they would chew out the bobbin case. It came with plastic bobbins for a reason. Don't use the metal ones if you want your bobbin case to last a long time.
I have an old Singer and it is all metal so using the metal ones in it is just fine. I also use the plastic ones in my Singer but doing the opposite of useing the metal in your Brother is not a good idea.