I have been shooting the FMJ's for around 6 months now and I have managed to break a few, mostly by hitting rocks. That said they have taken knocks that would have destroyed my old Axis ST's and still been fine.
I havent broken any in animals, as they have passed through everything they have hit.

I have bent one FMJ shaft so far, but that was by hitting it at an angle with another arrow.

They do slow the bow down a little with the extra weight though.