I came across some old forum post where someone claimed that the internal ATAPI Zip drives seem to be more well-built and reliable, that they've never had any issues with internal drives and it was only ever the external ones that seemed to die and exhibit click of death issues.
This struck me as fairly true from my own experience, as currently the only two functional Zip drives in my possession are the internal VST Zip 100 for my PowerBook G3, and the external VST Zip 250 FireWire, which pretty obviously uses an internal drive in a shell. Meanwhile all the broken drives I own are all external ones - all USB, at that.
Maybe the external drives were made of cheaper components? That would be absolutely foolish if true - you don't decide to make the drive that never moves and sits in an isolated environment out of better stuff than the drive that's gonna be tossed around and exposed to everyday wear and tear. Maybe it's just that because those internal drives were isolated and not tossed around that they've survived.
The click of death comes from the Zip drive heads falling out of alignment (and iomega not implementing ANY method of realigning them). So I guess the major design flaw in Zip was that the drive heads could be slowly jostled out of alignment from constant bumping and knocking around, and as long as you didn't do that, you'd be fine. So, you know, just don't move your portable disk drive around. Yeah.