[SOLVED] how common brand new high end mobos come defective?

Solution
Same answer, if it were faulty you wouldn't be relying on warranty but the seller.

Bad components certainly happen, but quite rarely. They are tested, boxed, and shipped. With hopefully the boxes designed well enough for the parts to survive the trip. There has to be some fundamental flaw built into the device that wasn't triggered by testing.

I suppose some bad soldering could lead to a component coming loose in transit.

And electronics can certainly fail on startup under harsh conditions. A common thing I like to do is let parts acclimate to an environment before turning them on. A motherboard delivered in winter shouldn't be unboxed for fear of condensation when exposed to warm air. Taking a near frozen CPU, slapping it in a...

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Same answer, if it were faulty you wouldn't be relying on warranty but the seller.

Bad components certainly happen, but quite rarely. They are tested, boxed, and shipped. With hopefully the boxes designed well enough for the parts to survive the trip. There has to be some fundamental flaw built into the device that wasn't triggered by testing.

I suppose some bad soldering could lead to a component coming loose in transit.

And electronics can certainly fail on startup under harsh conditions. A common thing I like to do is let parts acclimate to an environment before turning them on. A motherboard delivered in winter shouldn't be unboxed for fear of condensation when exposed to warm air. Taking a near frozen CPU, slapping it in a motherboard and turning it on immediately might result in thermal shock (bad example since just putting the CPU cooler on it would bring it to air temperature in a few minutes.)
 
Solution
Puget systems indicates that about 2% of motherboards fail initially:
https://www.pugetsystems.com/labs/articles/Most-Reliable-PC-Hardware-of-2018-1322/#Motherboard

I suspect that is about right.
The important thing is that such failures are found immediately before the pc gets into production.

If you look at verified owner user reviews on newegg, the frequency may appear high.
But those with failures are more likely to post a review than those who built without incident.
Then also look at the reason for a negative review; many are due to user error.