I have some pre-wound bobbins and they do work in all my class 15 machines. But the thread is really huge so I haven't found a need for it.
As for the plastic bobbins, I've had no trouble with them in any machine as long as I use the correct class bobbin for the machine.
I've also bought both the metal and plastic class 15 bobbins from Sew-Classic and fail to see why she doesn't recommend the metal ones. They've been very good and have worked in my machines.
Joe