Weather stations with multiple external sensors?!

You hit the nail on the head as usual :-). I also noticed the deviations in the relative humidity; according to the reviews, it's a bit of a lottery whether your own Netatmo measures somewhat accurately or not. For that, it's definitely too expensive for me!

Netatmo would be nice in terms of functionality:
- Temperature
- Rel. Humidity
- CO2
- Precipitation amount
- Wind (personally not important to me)
- all of this both indoors and outdoors

Other disadvantages: apparently(?) it runs through a Netatmo server. Does it require a permanent internet connection? That's not clear to me yet; that would be another exclusion criterion for me.

Klima Logg Pro by TFA seems more reliable at first glance. I haven't quite understood yet how the outdoor area is recorded.

I am also looking for a reliable sensor for room temperature and humidity. I have already tried several cheap devices, and the deviations have always been outrageous. However, I do not believe that better accuracy can automatically be expected with more expensive devices.

Klima Logg Pro from TFA seems more reliable at first glance. I have not quite understood yet how the outdoor area is recorded.

I simply attached one of the external sensors outside on the balcony (I am aware that it is always somewhat warmer near the wall than on the ground in the garden). The sensors transmit the data wirelessly to the display unit. The display unit also has integrated sensors and thus shows the data for the installation location. The external devices show temperature and humidity alternately on their own display and transmit the data to the display unit at regular intervals. How often these values are stored in the display unit can be set, e.g. every 5 minutes. The shorter the interval, the finer the resolution of the data that can later be transferred to the PC.

Regarding the accuracy of the humidity measurement: I tested all my 8 external sensors next to each other in one place and there was only about a 3% deviation in humidity with one external sensor, all others showed the same value after a short acclimatization. Whether the values are absolutely correct, I cannot say, but that was not so important to me. The fact that the values were almost all the same I interpret as a sign that the devices are delivered reasonably well calibrated. For even more accuracy, one will probably have to spend more money.

Another tip: I bought a second set back then, which was cheaper than buying 4 external sensors individually. So I still have a spare display unit that I am currently not using. Of course, one could also set it up in parallel at another location and teach the sensors there as well, then you would have 2 display units.

The best and most accurate devices are probably still those that work with chemical "tricks" (e.g. temperature measuring units with mercury).

It is clear that a high price is no guarantee that the devices measure properly. However, it is basically absurd that people prefer to spend money on attractive aesthetics rather than ensuring the actual function. A 10% relative humidity deviation is not measuring but guessing.

the best and most accurate devices are probably still those that work with chemical "tricks" (e.g. temperature measuring unit with mercury).

Apparently not, how do you come to this statement? Stiftung Warentest tested hygrometers in 2003 and found that electronic devices were generally superior to the others because mechanical ones are usually not properly calibrated. Among the electronic devices, the measurements of 5 out of 7 tested devices were good. The test is indeed old and whether the statement still applies today is of course unknown. The trend is often towards cheap products, but probably also with mechanical devices. There are various test websites on the internet that claim to have tested hygrometers, but what to make of that, well... Often, the devices from TFA are among the better ones there. And whether Amazon reviews give more insight is also questionable. At least I am satisfied with my devices and am also aware that they are not intended for scientific or expert purposes. Then you have to look at Fluke or similar brands, there is surely something for higher demands, but then it is no longer a weather station.

I am very satisfied with Netatmo. On all mobile devices, I have good access to all measured data. The most important function for us has become the CO2 measurement. Coupled with a Hue lamp, it signals to us when it is time to ventilate again. At a value of 1300 ppm, a lamp starts to light up red. Really practical.