The manual of my Philips Sonicare says, that the battery
is "not to be replaced", but on failing the whole unit should
be dumped. When I discovered that, I decided, never to buy
a Philips device again.

But this is true of all the rechargeable toothbrushes.

The Sonicare probably does a better job than the cheap Braun, but I'd be
reluctant to claim that as fact.