Actually, string theory deals well with the event horizon of black holes.<quoted text>
All three theory's run into troubles with black holes and Singularity's. so either all three are good theory's and Singularity's and black holes don't really exist that would kind of kill the Big Bang or black holes or Singularity's do exist and the singularity at the start of the Big Bang might have actually have been there then all three theory's have to be reworked.
But yes, there are difficulties when investigating black holes 8very* close up. At that point, quantum effects come into play and, once again, we simply do not have a quantum theory of gravity.
BUT these theories do quite well anywhere other than the event horizon and singularities of black holes and the beginning of the universe. They are quite valid *theories* about gravity and how the subatomic world works. No *observation* has ever violated the predictions of either. And *that* is the standard of science.
Here's a question: is Newton's theory of gravity a theory? Was it a theory 200 years ago? Was it a theory 200 years ago even though it didn't explain light? Or atoms?
GR and QM deal with different aspects of the universe. They are incredibly good at describing those aspects, each in their own realm. The problems come when *both* look like they should apply. Unfortunately, we simply don't have any *observations* of situations where both apply. Believe me, such observations would be a HUGE help in finding a more comprehensive theory. But, both GR and QM *are* theories right now and are even incredibly good theories.