Daycare has its uses, among them socialization, and there are decent daycares out there if one digs around a little bit.
However, daycare is generally not a place where a puppy learns good habits. Due to the general lack of oversight and trained workers in many daycares, problems with potty training, manners, hyperactivity, and even aggression can occur.
If you are going to be using a daycare, it is best to question:
1) Is my dog a good fit for daycare?
Some people leave their dogs at daycare not realizing that their dog is terrified of daycare! There are often dogs hiding in corners, shaking and afraid in a busy and overwhelming daycare environment. Make sure your dog is comfortable!
2) What are the qualifications of the staff?
Many daycares use workers that are minimally trained and underpaid. A good daycare might have workers that have been specially trained to monitor play, handle aggression and unwanted behavior in R+ ways, and may even have an actual trainer on staff or on call!
3) Is my dog well trained? Will their training suffer?
If your puppy or young dog has had very little or inconsistent training, there is a large chance that daycare will throw that all right out the window. Even if you are working with a trainer frequently, daycare may slow down progress due to the fact that the dogs are often left to run around with no structure.
A puppy with no or little potty training will most likely have issues with potty training unless they are part of a special puppy class. A dog with no manners will probably learn even worse manners. A frightened or scared dog will probably become even more frightened or scared.
These are all situational and not guaranteed to happen, but in general it is usually best to find specialized puppy socialization classes run by a trainer, work with a trainer to get through the other issues, or even find a good dog walker who can socialize and help your dog if you have a dog that you are not sure is a good fit for daycare.