Complex sentence: Many problems have emerged from the thoroughness with which the BBB does its job, especially the difficulty experienced when transporting drugs across the BBB thus making it difficult to treat diseases of the brain such as cancer, Alzheimer's, and Autism.
Split into simpler sentences: Many problems have emerged from the thoroughness with which the BBB does its job. Transporting drugs across the BBB is one of the biggest obstacles, because the thoroughness with which the BBB does its job makes it difficult to treat diseases of the brain such as cancer, Alzheimer's, and Autism.