The only downside I have ever found with the magnesium bar/firesteel is with a round version I had.....the magnesium was drilled for carry...which I did on a keychain and sometime during carry the firesteel dropped off the magnesium.

The glue bond has always bothered me and I will no longer carry such combo sets on a keychain or where the loss of the firesteel is likely.

I almost never ever bothered with the Mg shavings though.