Stress, pressure and fatigue.
A primary disadvantage of multitasking is that it promotes the illusion that people can do two things at once, and nothing could be further from the truth. Research by neurological physiologists and other investigators into the way the brain works confirms (not reveals - it's already been revealed) that the brain is a "one-thing-at-a-time" kind of machine. The strength of the brain is in its ability to switch very rapidly from one task to another. And because the brain is, in general, pretty good at switching, this gives rise to the illusion that people can do two (or more) things at once. They cannot, and nothing could be further from the truth. As the brain doesn't do two things at once, a driver is actually not in control of his/her vehicle for the time it takes to do use a cell phone or do something else. This creates danger beyond calculation. We'll never really know how many people are killed or injured in auto accidents (huge dollar loss aside) initiated by someone who is driving and "multitasking" on a cell phone or in some other way. Happens every day. It's happening now.
[This is being written a week or so after the collision of a commuter passenger train into a freight train in the Los Angeles area. The engineer was killed, but was texting up to about 20 seconds prior to the crash. (He was killed, along with two dozen of his passengers.) This engineer ran red stop lights and "engineered" the collision.]