They are also extremely expensive and use expensive esoteric batteries.

I like Fenix flashlights. They sell a lot that use normal AA and AAA batteries. They're also suprisingly bright and they don't lose brightness as battery voltage goes down with charge until the battery is almost done.

Yes, it is engine braking, and it is good. The same thing happens in a manual, but in a manual you have more flexibility in downshifting (which means you can use more engine breaking, but when done excessively you cause excessive wear).

The engine is meant to slow down the car (and ideally, but in many scenarios not realistically, do most of the slowing down). It is not meant to do it very fast (or come to a complete stop). That's what brakes are for, rather than excessive downshifting.