The word "uncontained" can be a bit misleading. The (thankfully!!) few big, bad nasty engine failures I've been involved in let lots of nuts and bolts go out the tailpipe. Because said scrap iron did not exit sideways through the engine casing, the failures were "contained" even though a fair mass was missing after landing.
A defined "uncontained" engine failure blasts bits out the side of the engine through the casing. This is obviously bad as the bits can enter another engine and turn a bad day into a nightmare.