I personally think it is difficult for any rescue to assess a dog 100%.
A dog in kennels often behaves differently there than in a home environment, and assessing a dog in foster really depends on the length of time the dog is is in the "foster" home, and then only a guide can be given.
We have had many fosters, and some of them are so stressed when they come it can take months (if not years)for them to show their true temperament, rescue centres big or small, just don't have the resources to keep the dogs that length of time.

It also depends on what the dog is subjected to in it's new home - it may have been fine in foster, but may respond differently to situations in another environment.
A difficult one really, as both the rescue and the potential new owner have responsibilities.