The thing with Nurseries is that there are very strict rules about what parents can or can't do. If they are late for picking the children up, there are penalties. If the children are ill, they aren't allowed to go. If the staff are sick, then someone else steps in. If parents don't send them, they still have to pay. All these things mean that parents can't take advantage, know exactly where they stand so the staff are fully protected from emotional demands that grandparents aren't.
There is also an understanding that the children are properly insured, there is appropriate supervision levels, the children are properly assessed and reported on with a scheme of work, etc. The social mixing is also great for them. It is also far easier for the staff of a nursery to inform the parents of behavioural issues or social integration problems than it is for a grandparent. My DGS was autistic but the parents dismissed anything I said but realised that there really was a problem when the Nursery Staff asked them if they could get the SEN staff involved.
I started looking after my grandson for 2 days a week when he was 9 months old and, although I think I am quite a tolerant person, I found myself being highly irritated by his parents when they came to pick him up and it was obvious they had been shopping or had gone to the gym first. Admittedly I wasn't being paid for my services but I just felt that I was just being used especially when they sometimes took a day off but I was still expected to look after him, sometimes without knowing they weren't at work. We did manage to sort it out but there was a time when I thought it was going to end up in an argument which I really didn't want. It was also exhausting, cut across my week and not as fulfilling as I had thought it would be so when I wanted to cut my days to once a week, it was very awkward.
You may be a very different person to me but one of the things I also miss on the days I have the grandchildren is adult company. My husband disappears and all my peer group don't really enjoy the company of small children for very long.
Obviously, if you've taken careful consideration of the pros versus cons so you still want to go ahead, then go for it and have a great time really engaging with your grandchildren because that is the best pro there is going.