I use a lot of rechargeable AAA and AA batteries. They have lower voltage than alkaline batteries (by design apparently), which is not normally an issue, but sometimes is a deal breaker. E.g. my thermostat did not like lower voltage AA batteries and shut down prematurely a number of times.

There's challenges adopting standardized rechargeable batteries, e.g. trying to recharge alkaline batteries risks fire/explosion (and you know that will happen far too often given the number of people out there), but if we have had standard battery sizes, voltages, and capacities for rechargeable batteries, things would be so much better.