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. The difficulty experienced with transporting drugs across the BBB makes it difficult to treat diseases of the brain, such as cancer, Alzheimer's, and Autism.