I fostered for 3 years while l lived in Texas - HUGE unwanted dog problem there. I loved it. I have 2 of my own, and I fostered about 10 dogs over the 3 years including a litter of abandoned puppies for a couple of nights while they arranged transport to a more experienced puppy fosterer.
My last one, Muffin, was adorable - but had the most awful hip dysplasia which no-one had noticed because they were all falling in love with her eyes and her smile and her waggy tail at the pound. She should never really have been pulled, but the rescue charity I fostered for started a special fundraising appeal, and with me nursing her, she had two hip operations 12 weeks apart, and was with me for a total of 9 months instead of the usual 10-14 days!! They did check that I was able and happy to nurse her.
Once she was finally ready for adoption people asked me how I could bear to part with her. I admit I cried buckets and I still get this squeeze in my chest when I think of her, but she went to live with the most marvellous couple up in Connecticut. The lady had fostered and raised 9 children and her last was about to go off to college, so she was looking for a companion. Her health was declining so no more children for her. She walked with a zimmer and couldn't climb stairs. She wanted a dog who would just shadow her, ride in the car with her to run errands, and be showered with love.
It was the perfect home. She sent me photos after Muffin got there - the two of them walking very slowly to the park, where Muffy got to run off-leash and her favourite thing of all, to swim in the lake, which meant she gets the perfect exercise for her hips, and doesn't tow her new owner around with too much energy.
Matchmaking like this is the perfect ending to a foster. I highly recommend it.
Personally I've never had a foster failure as 2 dogs is enough for me - I love having a 3rd occasionally, but I liken it to having kids for a sleepover - you love the kids, you love the fun and chaos, and then you love it when they go home the next day and the house is quiet again 