Cats, no. Cats can be spayed and neutered early. I personally think 8 weeks is too young, but shelter medicine is different than general practice.
Evidence is mounting that large breed dogs that are altered early may be at higher risk for various cancers, orthopedic problems and other issues. My dog has a couple issues that letting her have one complete cycle will fix. However, it does increase her risk for mammary cancer.
My other concern is that I have working dogs. Altering them early changes their musculature. I want them to attain their full mass as much as possible.
I do NOT recommend delaying altering for all owners. It's a very individualized concern. Many of my owners cannot deal with intact dogs...we have too many unplanned pregnancies as it is among our patient base...so I am telling many owners to spay and neuter early. But for some, it can and does work.