A kitten that is taken away from mother or siblings at too young an age can have aggression problems throughout life. Mom cat teaches the kittens in a way most humans can not. I adopted a 1 week old kitten when his mother was killed. He was so aggressive, I finally adopted another cat to teach him the rules. It was either that or have the cat put down. He did get better, but never became a loving cat.
I have done both clinic spays and hospital spays. It seems just the luck of the draw, but the only cat I lost and the only cat to get sick were both done at the veterinary clinic. I don't blame the vets that did the surgery, sometimes things just happen. But, I would not avoid a clinic out of fear either.