I've been doing a bunch of investigative work into why so many 27's seem to be giving users problems. It turns out that the 27 uses a different technology than the 21, 22 and 25 do. (I'll get the to 25C and 29C later).
The 21, 22, 25 all share a common power supply that outputs +6.5 volts and -12.5 volts from the 2.5 volt batteries. The 27 uses a very simply and cleverly modified power supply to output +6.5 volts, +12.5 volts and -3.3 volts. The +12.5 volt supply sharply contrasts to the -12.5 volt supply in the other Woodstocks. The -12.5 volt supply in the earlier calculators suggests that they use PMOS technology, while to +12.5 volts in the 27 suggests NMOS technology.
At the time the 27 was being developed NMOS was coming into favor with LSI IC manufactures because it could switch faster then PMOS. My guess is that HP decided to get its feet wet with NMOS in the design of the 27. Unfortunately, I'm not sure that the IC manufactures were very far along the learning curve in using NMOS and didn't produce very long-term reliable chips for HP. Also, unfortunately the use of NMOS verses PMOS makes using a chip swap approach to fixing 27 difficult because one has to get the chips from another 27 -- not a cheaper alternative calculator. (Actually, one of the two display/keyboard driver chips is the same as the older 21/22/25.)
Interestingly the 25C is identical to the 25 but uses an even more advanced technology, CMOS, but just for the RAM chips (it uses 2 instead of the one in the 25). The 29C is an entirely different calculator internally. It seems to use CMOS for the processor as well as the memory, uses several 8-pin ROMS (like the Spice series) instead of the one 16-pin ROM in the other Woodstocks and has a completely redesigned power supply. The CMOS processor is the reason why these machines (and the 19C) remember their stack when switched off.
All of the above was gathered by looking at the internals of these machines and making some measurements. So I might be somewhat off -- or even very far off -- in some of my conclusions and assumptions. If anyone has some more knowledge about the Woodstocks I'd love to learn about it.