I have some mutually contradictory information about the longevity of NiMH battery packs, as used in non-plug-in hybrid cars (mostly by Toyota / Lexus, but there may be other brands as well).
On one hand, Toyota offers a 10 year / 350 000 km warranty on the hybrid battery pack at least in Finland. Also, Consumer Reports has a 206 000 mile test of the Prius, noting that the performance and fuel economy was identical with a 2 000 mile driven Prius, thus making it likely the battery was in a good condition, although this test was on two about 10 year old cars, thus testing only amount of miles driven and not amount of calendar years.
On the other hand, in a comment to the post Toyota Auris Hybrid sometimes is reluctant to enter READY mode, Paulster2 noticed that "you can purchase a remanned Dorman Battery Pack for a 2016 Camry off of RockAuto".
So, in general, how well typically do the NiMH battery packs last as a function of time and as a function of miles driven? Let's assume an intelligent control computer here, one that shallow-cycles the battery to obtain greatest possible longevity. I am at least aware that some Honda hybrid vehicles have had suboptimal control algorithms, thus resulting in early battery failure.
How common is a battery failure on hybrids when compared e.g. to a transmission failure on a car with conventional automatic transmission, or to an engine failure on any internal combustion engine powered car?
Let's make it clear that we're discussing the same thing: this question is concerned only with the hybrid battery pack, not with the auxiliary lead-acid 12V battery that will surely fail in a short amount of time.