More analysis (including calculations) from another YouTuber: https://www.youtube.com/watch?v=FuQ_AAkkgIg

Is there also some written information to be found somewhere?

"We're sorry, YouTube is currently blocking us from fetching subtitles preventing us from generating a summary for you. We're working on a fix!"

That almost sounds like a bug that could be sold as a feature "the shower with that extra wake up jolt!".

Well... Not gonna lie, the few times ive been cycled by HV I was very awake afterwards... And then really sleepy...

How did it happens?

The major difference being that the UK-approved on is a ceramic heater: a resistive wire suspended in ceramic, encased in a grounded metal sheath (for transfer of heat). Regulations demand fixed cabling, of appropriate size, all on a quick-acting 30mA-leak RCBO at the consumer unit. Ignoring regs is a good way to lose your accreditation, get sued into oblivion so most electricians follow them to the letter and beyond. Paperwork to that fact is required for the installation.

If the wire touched the metal, or water got into the element and bridged the circuit to earth, the breaker would cut out immediately.

The suicide shower relies on the exposed resistive heater having a lower resistance than the water, and then a functional earth-leakage breaking device.

In perfect conditions, they're probably Safe Enough™, but there are so many failure modes. Earth failure (corrosion? never installed? fast enough?), dodgy water chemistry, exposed cabling, an exposed join because the included terminations are only ~15cm. Often installed in countries without rigorous electrical safety standards. Just a bad idea.