under biggest challenge for rescue and foster is: Finances and space.
If you do rescue work, you often times have to cover immediate vet care on your own- the stuff that needs to be done asap, like deworming, distemper, uri symptoms, and dealing with fleas and also the unknowns (I had to deal with lung worm once with kittens I'd trapped and of course paid for). This is costly. Then the challenge of finding a shelter that will take them to find them a home. ("Oh you found them in xyz town, no we only take strays from pdq towns sorry" or "this thin area on the ear might be ringworm, bring him back in 3 weeks", or "His eye looks a little watery, bring him back when it's been treated with vet paperwork saying he's not sick".
God forbid ringworm hit your house ($$$); not just to treat but it means you have them longer and therefore more litter and food and possibly another bout or two of uri or eye infections... you never know with these strays whose immune systems are so compromised to begin with.
Then, if you do get them into a foster home or shelter, they still may be put down.
With fostering from a shelter or a rescuer- again Finances. Ålso space. Same as above. But if you are fostering for a shelter, maybe they are paying vet bills up to a point? However, I have done fosters for a shelter and the fosters never went in the shelter because guess what? Always full! Did they care? NO. Guess whose cats they became? Mine. And another rescue group was kind enough to place all but one that I kept.