To blanketly call the 'sliding X' a death knot is pretty rediculous, as it certainly does have uses. So much depends on the purpose of the anchor and the terrain.
The main purpose of a multi-pitch belay anchor is to protect from a factor 2 fall. In this instance, full redundancy should exist in all parts of the anchor, and it should be pre-equalized, no sliding x here as the force on the anchor will always be straight down. (redundancy should always exist in any part of the system that is 'whole party dependant'. Someone mentioned the rope is not redundant, neither is your locking anchor carabiner. should either one of those fail, only one climber is gone, not the whole party, should your single sliding x sling fail, the whole party is history.
So use a cordelette or double length runner with a knot or a rabbit runner with a knot for most anchors and you're set.
As for shock loading enough to break the sling or biner, it can happen, especially if compounding the situation with other mistakes such as tying in with a static link such as a daisy chain or runner instead of the rope, and/or alowing slack in the system to further shock load things. According to Petzl, a drop of around 7 feet on static cord can result in forces greater than factor 3, which can and will break biners, webbing and harness'.
For rappelling anchors or end of climb anchors, it would be impossible to load any part of the anchor enough to break anything, providing the bolts and/or gear is good, sliding knot here, sure, and for equalizing crappy gear placements. (I guess most of us, I hope, rely on being able to work out tricky gear placements and not rely on screamers)