mm43: I appreciate the additional input but only a couple of the points as noted below might argue for NiCd vs NiMH for aircraft use, so I still wonder what really drives the designers in that direction.
"High self-discharge" of little concern in machines that fly much of every day. Plenty of ways to manage exceptions.
"memory effect...not as much as NiCads" is a plus, when comparing to NiCads
the bean-counters hope that ops avoids "long-time-storage" of their fleet
"high discharge" performance would need to be evaluated. Maybe that's a problem.
"less tolerant of overcharge" should be managed by design
"safety vents...as with NiCads" is a null argument in this comparison
"coulombic efficiency" - NiCad is better?
"capacity ... not necessarily all available" - NiCad is better?
"Cell voltage is only 1.2 Volts" sounds exactly like NiCad
"[worse] than alkaline primary cells" makes no sense in the comparison
"Lanthanum [is scarce]" doesn't seem to be an issue for the millions [?] of NiMH cells sold in the consumer market. (I understand that rare earths are in fact present in the U.S. but cheap Chinese supply made mining uneconomical a few years ago.)