The Prius only needs a tiny amount of range in their NiMH battery (probably just a mile or less) to be enough to support acceleration from stops, and to accept regeneration. There’s a reason why all modern EVs with any reasonable range (even hybrids like the Chevy Volt with 37-50 mile range) all use Li-ion batteries and not NiMH batteries like the Prius.
The Li-ion have higher energy density when it comes to weight and space compared to NiMH. You can’t just simply say one battery chemistry is as good or better than another battery chemistry without factoring in what application that battery chemistry is being used for. For the application of the Toyota Prius as a hybrid, the use of NiMH is good enough so the use of Li-ion is not necessary. In that application, the NiMH has the advantage of a lower cost.
If rechargeable HAs use NiMH chemistry in the current shape and form of the HAs instead of Li-ion batteries, I’m sure the HAs won’t last you the 12-16 hours of use daily. The same with the ConnectClip. If it had used NiMH batteries, you would have had to charge it a lot more often than its current form using Li-ion batteries. May not even last you all day before you have to recharge if it were NiMH unless they make the ConnectClip twice the size or something. Then nobody would want to buy it if it were too big to carry around in their pocket or wear around their neck.
And it’s not just the weight and size disadvantage for the energy density. NiMH basic cell voltage is only 1.2V, while Li-ion basic cell voltage is 3.7V. If you have designs that require a higher operating voltage, let’s say at 3.6V, you’d have to put 3 NiMH cells in series to achieve that voltage, while only 1 Li-ion cell alone can deliver that much voltage already. Surely you can redesign things to work with 1.2V, but perhaps at a higher cost than a 3.6V design.