I think it depends on how skilled and knowledgeable the parent is about child development, but also on how good s/he is with knowing how to build the right kind of relationship with the child. If a parent is young or doesn't have a lot of experience with/knowledge of children and child development then I think the child may benefit from the right kind of care-giver.
If the parent is very skilled and has a solid understanding of children (but also of human nature in general) then I don't think anyone else ever makes an equal substitute.
I think it also depends on how young the children are. Babies and children under three or four really benefit from having at least one loving, capable, parent devote their time and attention to them; and build that bond between adult and child. Children older than that reach an age where they want to expand the world beyond home and parents anyway, so that makes a big difference.
Whether hiring child-care is "good" or not is very much based on the individuals and the child-care person/people in question. None of what I'm saying is a blanket statement about being "against" getting child-care help. Sometimes, too, when mothers (or fathers) have no choice, it has to be good enough.
I, personally, would not have trusted my child's early (under four-years-old) cognitive, emotional, and social development and days to anyone but me. As it happened, I had a husband who worked and earned decent money; and I had a lot of experience with, and exposure to babies, and children; as well as having studied up on child development (before I ever had kids of my own). On the other hand, if I had to choose between day care/full-time work outside the home and living in a dangerous neighborhood and had deprived kids (because of lack of money), I would have gone with the less ideal thing of leaving them while I worked. As it was, I changed my work situation and worked part-time from home when the three of them were little.