FWIW I've never seen a seat-post fail/break... But they like to get stuck REGULARLY!
What I have seen a few times over the years is the frame failing in that area, broken seat tubes especially. Just a smear of grease will stop the bad stuff happening but often seat-posts are forgotten during regular maintenance. Carbon grip paste works well in Carbon frames.
(the unstuck seat-post gets binned)
This is the seat post area of a new frame being built by Darrell Llewellyn McCulloch. Note the duel seat post binder. This is done to accommodate carbon seat-posts. He's built literately thousands of bike frame and other components over the years. Darrell sells these via his web page:
Llewellyn Custom Bicycles Home
