My problem with kaguya is not that she existed but that suddenly near the end of the entire series we learn of her existance (much like Hamura) only to have her appear when Madara, the villain we knew existed since the begining of the series, had reached his Apex only to be stabed and betrayed by something that was believed to be his will, turns out to be Kaguya the woman we just learned about 5 chapters prior.
At that point any sense of danger that Naruto or Sasuke could die left us. Why? Because Madara killed them both once before and now that he was even stronger there was a real chance that he could do it again. But once Kaguya appeared that sense of danger was gone.
Then letting Black Zetsu talk for her and have her only talking about how all chakra belongs to her made her feel one dimensional. We had no time to really get to know her motives. We were left wondering:
what was the pivital point in her life that led her to eat the fruit in the 1st place? How did she find it? What drove her to want to procreate? What was the woman like before all this happened? How was she defeated by Hagoromo and Hamura?
All of these things should have been brought to light way before they did.
The build up to her appearance wasn't there, and when she finally did show up she felt misplaced.
Her character had potential but I feel like the series should have ended with Madara and the movie should have dealt with Kaguya. She seems more like a movie villain then the series villain.