The steel type would eliminate any shatter risk, though in my experience the plastic body of the ordinary ones contains the remains of a broken filler reasonably well. If you are unlucky enough to de-pressurise coffee soaked charts will be the least of your problems!!
Ghengis, I think, anticipates a second problem with the pressure inside the flask forcing out the contents through the stopper. Keeping upright, using a good quality flask with a rotational rather than a push button stopper will probably deal with this.