"or in 2.6 million units one will fail in an hour ?"

More likely !

In any case the cost-reliability-margin matrix is highly subjective both from the user's point of view and the designer's ( supplier's) point of view.

And the rules of the game vary with the domain - what goes in consumer field may be a non-starter in the industrial field and if you enter aerospace or medical diagnostics - most of the components that are normally used may not even qualify.

I for one belive in educating the unwary customer about the pros and cons of the proposed or requested bells and whistles. Home work done upfront before hitting the drawing board has ensured many a project that are closed out peacefully.

And nothing to match experience - when it comes to deciding where to draw the line.

Raghu