Complex sentence: They believe the general population has accepted this hypothesis into their understanding of mental illness uncritically; other critics have pointed to the lack of an established chemical "balance" (without which the notion of an "imbalance" would be meaningless).
Split into simpler sentences: They believe the general population has accepted this hypothesis into their understanding of mental illness uncritically. They have pointed to the lack of an established chemical "balance" (without which the notion of an "imbalance" would be meaningless).