The usual failure mode is bending, not usually breaking or chipping, but it can and does happen.
I have seen chipped Glock extractors, but never a bent one (that I was aware of anyway).
Is this just in reference to 1911 internal extractors?
I don't know much about them.
The Glock Extractors are MIM or cast then hardenend/annealed. I have never read an analysis done on them, so I don't know what steel alloy they are made of and if they are through hardened or just surface hardened.