As for Umineko, EP7 made me cry too. Because it was then that I finally realized that (Umineko EP7 spoilers) no matter what universe, Yasu/Lion will die. Nothing can be done to prevent the death of Yasu/Lion, nor the deaths of the Ushiromiya family.
There’s an extreme sense of powerlessness I felt when I came to that realization. Nothing is as sad as realizing there is nothing you can do to change the outcome of a tale, especially one as tragic as Umineko.
As for Higurashi, a similar thing happened in Minagoroshi. (Minagoroshi spoilers) watching everyone being shot in the head, one by one, by Takano. It was heartbreaking to see Rika say over and over that if they just had one more person, the outcome would have been totally different. Again, I felt powerless and sad during this scene. Because I had grown to love these characters, and rooted for them all this time. Yet, I still couldn’t save them.
I think these sad moments truly show how good Ryukishi is at writing emotionally captivating material. I often point to them when praising him as a writer.