I stopped fostering for 7 months because my apartment was infested (I feel, through contact with a foster cat,) with the kind of fleas that bite humans' ankles and feet (for a month before I was able to successfully treat them,), and the shelter (that I worked with at that time,), denied that the fleas came from the foster cat (that I fostered from them). For the previous several months, I hadn't bought any second hand items, had any animals (I have no animals of my own,), or exposed myself to any other source of contagion (so, in my mind, it had to be that cat). Because of that occurrence, I now give a flea-bath to every animal prior to crossing my threshold, because I need to protect myself first, prior to fostering (and giving a flea bath is one of the ways that I now do that).
Another issue is that underage kittens, especially ones that are orphaned and/or sick, need so much extra care to keep healthy. If you work full-time, underage kittens can be a second full-time job unto themselves (which is exhausting and overwhelming sometimes).
Also, a quiet little queen who keeps her kittens feeling secure and therefore quiet is a rarity. People who live alone are not as commonly people who can endure the extra stimulation of a raucous cat family. The kitty litter, the messes, and the havoc that they can create can be too much for a quiet person. Even something as simple as any kind of health aide, i.e. CPAP machine/plastic tubing, is vulnerable to a curious kittens' claws and teeth, and these are not inexpensive items to replace.
Fostering is a labour of love. Fostering requires time and commitment to do well. But it can be worthwhile as, even for a short time, those kittens give back love and life to a lonely person. There are too many lonely people in the world, in big houses and with no one else. If there is a way to reach out to those people, as, two kittens can be quite content by themselves for eight hours in a day, as long as they are in a safe environment. Then, allowing the love the kittens give back, can be its own reward.