I cannot say that they all turned out mentally well, but they had a good start. I was born with intestinal jaundice due to my mother having German measles when she carried me. Fortunately penicillin was readily available in the early 50s, so I survived that and grew up a healthy boy. Now I cannot speak from the perspective of a parent about raising a large family, but since I was part of one I have some insights. First of all, in modern times I think it is a bad idea.
Children have a greater survival rate than in the distant past.
Plus, this world's population has tripled since my mother's birth from about 2 billion to over 6 billion souls. So, yes, you can blame my mother for helping to over populate this world. I believe it also causes a lot of stress for the whole family to have one baby after another. Children need a lot of attention. I personally felt lost as a child in this zoo of siblings.