If you want to be a pessimist, plenty could go wrong. The mom could develop an infection, reject them or even develop a taste for them. the pups could get accidentally squashed by the mom, pushed aside by their litter mates and slowly fade away. They could get dehydrated real fast from diarrhea caused by coccidia, giardia or many other undiagnosed causes and by the time you notice it, it is too late to save them. These are just a few of the possible pitfalls in the first week that popped out of the top of my head. I am sure I am missing plenty more.
But if you want to be an optimist, usually if you watch them like a hawk, keep their whelping box clean and warm, their mom well fed and calm and able to keep them alive and well for the first week, their odds of surviving increase dramatically with every subsequent week that passes after that first critical week.
If I seem to have a superiority complex, it is because you make it so easy.