I don't think impurities/flaws are necessarily the only explanation for this particular failure.
Cavitation and/or galvanic corrosion (which I'm positing would be concentrated on the top(outer) curves of the neck) plus clamping force (inherently uneven with a worm clamp and note the location of the worm in this case), could easily have caused a crack to start at the end below the clamped hose and then propagate down the neck to the base.

Just a theory.

 

comparatively speaking to the stock aluminum necks i have never seen one fail, i have seen them develop rather severe buildup and corrosion though from old unchanged coolant that was several years old. i'd attribute that more to neglect than anything else. i'm a bit overzealous with worm clamps too, i break them quite often but never an aluminum neck even in craptastic shape.

seen a number of the plastic ones fail from the worm clamp issue putting stress on a particular point. also see the plastic ones develop hairline cracks from the base.

if in doubt do the hammer test and see which one survives.

problem i mainly have even with the new OEM plastic necks is that most of them were made 15+ years ago and have been sitting on a shelf for that period of time. even OEM coolant seals have been known to have this issue and break apart before even installing them into the engine because the silicone has hardened with age. which is the lesser of 2 evils? plastic doesn't have the corrosion issue but it does get brittle with time, otherwise i wouldn't mind them, i mean most have lived long full lives for 20+ years before they failed but these days even brand new OEM parts cannot be expected to live the same period of time because simply most of the parts are realistically not new anymore but are new old stock that were made many many years ago.