It seems that the standard rated power of Brazilian showers is 5.5 kW. But then again, Irish weather seems harsher than Brazilian weather.

Here it's quote common (if no cheaper method of central hot water is available) to have on-demand water heaters like https://www.clage.com/de/produkte/durchlauferhitzer which usually have a maximum power output of 21 kW, so they aren't just used for showers, but can also fill up a bathtub quite quickly. They also don't hang inside the shower, but on any nearby wall.

They're connected to three-phase power with a 32A fuse on each phase.

The 127 just heat less. Max 5500W. The 240 can go up to 7500W in some versions.

The tanks generally also have a heat exchanger within that's hooked up to the wet central heating system, that way you can have 1 boiler so both central heating and water heating.

A very common (albeit somewhat old-fashioned) plumbing setup in this part of the world is an 'open vented' system where hot water is supplied at comparatively low pressure - a cold water storage tank in the attic provides a pressure head of about 0.5 bar to a hot water tank, which feeds the hot taps. That's plenty for hand washing, but not really enough for a shower.

Whereas the electric shower takes mains pressure cold water only - giving you a shower without needing to replace or upgrade your hot water / central heating / boiler system.

There are other options (such as a pressure-boosting pump on the hot tank outlet - or replacing the tank with something newer) but changing between types is difficult enough it would usually only be done as part of a complete bathroom refit, which can easily cost $5,000+