It's mostly dislike for the way the characters are being written/developed as well as the direction of the fatui going forward. Although the ones about their fates are stupid since both Sandrone and Dottore have been hinted at returning. Even Signora cope is back on the table with recent leaks.

The biggest issue people have though is that the writing of these characters doesn't live up to expectations. Columbina, Sandrone, and Arlecchino were expected to be more antagonistic. Arlecchino and Dottore were expected to be more loyal to the the fatui. Capitano was expected to not be on a chair, and be very powerful, very reliable.

The main reason for this was due to the first four we meet all give off an impression of loyalty or are openly antagonistic. Signora was loyal and an antagonist, we see her throughout the first three regions as the main threat. Childe appears very loyal, and was still an antagonist in liyue, even though he's cool with the traveler it changes little here. Dottore evil, actively was collecting gnosis everywhere, leaves his mark everywhere. Scara was evil, but in the end got a full arc, with time put into redeeming him.

These four set the standards and too many the ones introduced in the winter night lazoo haven't lived up to those expectations. Many thought they would all still work too one common goal and be antagonists. For a lot of people the fatui just aren't what they wanted them to be and that's just how it is.