Stress levels in unsettled babies and toddlers is likely to be much higher. However, if the nursery has good, consistent care givers the baby does learn to trust and be settled with them. Babies also make friends with their peers from a very early age and this friendship can follow them on into school and beyond if they stay in touch.
My main concern with nursery type environments for young children (under 3) isn't so much about the care givers. In the majority of nurseries the children are cared for by experienced, well trained staff who do care for the children in their charge. It is more about the lack of access to the outside world. Children who spend significant amounts of their waking hours in a nursery setting aren't getting to see the world they are part of. They aren't seeing shops and parks, adult environments and chores, they are seeing the inside of a space that is set up for children of their age and although it means they can freely explore this space and the resources in it - which is a good thing - they are also missing out on what grown ups do. They aren't learning to cook and wash up, do laundry, go to the shops, DIY, tending the garden, unless it is only play and not real. They are following routines and structures that are fixed and less flexible than they would be out of that environment because there is no option for one child to have lunch at 12pm and the next at 1pm because it simply isn't practical. They have to follow a routine that is set for them.
Personally, I think children who spend full time hours in nursery based childcare are very advanced in some areas. They are quite independent and they have learned a lot academically. However, I also find that they are very peer orientated, they don't really pay so much attention to adult requests and interaction as they do their peers. They make less eye contact with adults than children who don't do full time hours. They can push boundaries more within the setting, they can be less confident on day trips away from the setting and more apprehensive about change.
My personal take out from this is children who do the 15hrs at age 3yr + benefit enormously from nursery. Children who are babies and 2yo can be less settled but can make good attachments and friends in their peer group. All children will make good academic and school readiness progress. Too many hours in childcare can start to become detrimental to their understanding of the wider world and societal connections.