It’s dark so requires torches (more than one as a backup) and very easy to get disoriented. You can easily get lost and run out of air. Risk of being blinded by silt even with a torch, leading to more risk of disoriented and getting lost. If anything goes wrong such as equipment malfunction then you don’t have the option of going to the surface as you do in open water (albeit with the risk of a bend). It’s often cramped with places to get stuck, snag equipment, or get tangled in your guideline. There are sharp rocks you can hit your head on.