Not being a hardware guy, I can only offer my nearly 26+ year experience with HP calculators. Among the ones I have in my collection:
Voyager series (HP-10C/11C/12C(USA)/15C/16C)
Most resistant to anything and everything, nearly
impervious to use. Never had one broke, all have withstood
heavy use with no visible effect. A particular HP-11C I used extensively for a year in the Sahara desert, while I was in the Foreign Legion, and 22 years later it still looks
and works like new (now running in its 3rd set of batteries in 22 years, expect them to last till its 30th anniversary at the very least).
HP-71B
Another brick of a machine, I own three and all of them are immaculate and absolutely as new, one of them despite heavy use in less that safe environments. Never failed
despite their age, seem unaffected by use or time.
HP-25C
Apart from the battery pack, which I had to rebuild, same as the previous ones: impervious to everything, looks and works as new (acquired in 1976)
Pioneer series (HP-32S/32SII/42S)
These models have also never failed at all, and they all are as new, but they are less old, and I haven't used them at any length, so I can't really compare them to the above models. They do seem quite solid and reliably built, indeed, if somewhat less than the Voyagers, the 71B, and the Woodstock series, not to mention the Classic series (HP-67, etc).
HP-41CV (fullnut)
Another model absolutely brand new. However, I've used (very extensively) a large number of HP-41C/CV/CX models, and while it's a great machine, I always had the feeling that it wasn't as solidly built and durable as, say, an HP-67, which I owned previously. Perhaps an example will help: if you take an HP-67 in your open hand, and strongly close your fist around it, as it to crush it, you'll feel that it does not crack or make cracking noises, no bending, it doesn't noticeably yield, you'll probably hurt yourself if you increase the pressure.
But if you try to do that with an HP-41C instead, you'll hear noises, you'll feel it bending appreciably, and chances are you'll actually damage the machine while applying only a moderate amount of pressure.
Spice series (HP-34C, etc)
The flimsiest of them all. However much I like these
machines, their looks, their display, their innovative and
advanced functionality, I can't help but feel that their physical casing is the worst, leading to many hardware failures, most specially non-working keys, etc. I think
that HP tried to cut costs, but they simply overdid it and
these machines aren't very durable or reliable at all, which probably explains their low popularity (except for the HP-34C, fortunately).
Finally, a special mention for the battery door and battery compartment of such models as the HP28S, which are the least durable and worst design ever (the battery doors and compartment, I mean) in any HP machine whatsoever, and the Chinese batch of HP-12Cs, which have the most unreliable keyboard ever.
Best regards from V.