Apple marketing led people to expect unicorn-like smooth perfection:
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While in real life, the Taptic Engine looks, as you said, more like a simple, inexpensive, thin metal, linear motor, with ventilation holes. Which is, after all, exactly what it is.
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To me, the Sport's bracket for the Digital Crown is also a bit cruder looking than expected.
But as you said, the outside is nicely done, and that's what matters to most people. I think Apple has moved on from the "make the backside of the drawer look just as nice" era of Steve Jobs.