with / without central ventilation system experiences

33% is, if correct, not good or too dry.

However, this cannot be changed without moisture recovery (or other measures). Although percentages of +/-40% are also achieved in a normal apartment without any controlled ventilation or other things in winter. That is normal – you can only reach 50% there with additional measures.

That's true, which is why the numbers of respiratory diseases also regularly increase sharply in winter.

per central controlled residential ventilation+enthalpy exchanger+activated carbon filter box in front of supply air distributor

This way, humidity is optimally controlled as well as possible (we are currently at ~40%) and you neither forcibly bring cold nor chimney smoke-laden "fresh air" into the house. I would implement it this way any time again

My brother-in-law (timber frame house with EPS insulation, foil, controlled residential ventilation without enthalpy exchanger) said the other day that he currently has 20-25% humidity and - no joke - he is getting cracked to slightly bloody hands.

20 - 25% is already extremely dry and certainly unhealthy. An enthalpy heat exchanger for about 800 EUR would mitigate the problem, but probably not solve it. In other words, you won’t reach the optimal 45-55% even with it.

With central controlled residential ventilation without the exchanger, we currently have an average of about 35-40%. In some rooms even only 32%. Certainly not optimal either, but almost "normal" in winter, independently of controlled residential ventilation.

In our house, controlled residential ventilation is among the most appreciated features of the new build. We were really looking forward to it beforehand and love it in everyday life :) Therefore, for us it also applies "never again without."

By the way, ours usually runs at the medium level and is NOT audible except for a single valve (where I need to adjust the settings once). Especially in the bedrooms, controlled residential ventilation is a benefit and ensures constantly fresh/clean air without being noticeable! And I also claim that especially here the argument about "open doors" probably does not apply, since most people probably close their bedrooms at night!?

Dry air does not come from the ventilation system but from the act of airing. I can only recommend everyone to simply play around with a corresponding calculator. Air at 0 degrees with 100% humidity comes down to 28% when heated to 20 degrees. The reality is even more extreme, as you rarely have 100% humidity outside. Without ventilation, you just don't notice it as much because you air out far too rarely ;)

Without ventilation, you just don't notice it as much because you rarely ventilate ;)

That may be true and probably correct, but controlled residential ventilation actually "exacerbates" the problem of dry air in winter :)