My local shelter was glad I did not bring the kitten back. She did not consider me a failure. If they had a home for her, then I could have taken her back as hard as it would have been for me to do. At the time they had almost 17 kittens under 14 weeks old. The chances of her getting a home were slim. And if she did then it was taking a home from another kitten. For that reason I just could not make myself taker her back.
I would have been able to do so if there would have been a home available to her. At least I like to think I would
I think if you trust the shelter's policy on placing animals, then it is easier to take one back. If you know the shelter is going to ensure a forever home then you do not have to worry about him/her. But if it is a shelter that is first come first serve with little to know adoption procedure then I think it would be more difficult. Also is the pet you are fostering one that will have a lot of people wanting it when he/she goes back. In this area small dogs are in high demand, and very seldom seen at shelters. Lab and shepherd mixes on the other had are everywhere.
OP maybe you could talk to some of the people who foster for the local shelters in your area. They could maybe really give you some firsthand information and information that related more to your area.