A living teacher can associate her experience with the facial expression and body language of a student. The teacher can understand sentence fragments associated with gestures, as well as slang. The element of empathy and play are important intangibles in the teaching-learning relationship. A computer can do none of these things, and if they are forms of communication upon which the student depends, then the absence of a living human teacher can leave the student stranded.
If the goal is to teach a student how to properly answer a question for which the answer is predetermined, a computer can do that very well. If the goal of learning is to knit a cap, paint a picture, play the tuba or drive a car, then the computer cannot be on hand and attentive while the learner is actually attempting to practice those skills. Computers can transmit information about any of these activities, but they are not a substitute for a living mentor.
Anyone who is untrained or has little aptitude on a computer is at a significant disadvantage when it comes to computer-assisted learning. Moreover, if the equipment or software is faulty, it becomes a roadblock to the progression of learning. This technological dependency means that frustration with the technology -- for whatever reason -- interrupts the learning process, and can stop the learning altogether. Technological dependency is associated as well with issues of cost and equity. Schools or learning environments -- such as homes -- that do not have computers are at a disadvantage.
Computers hold a certain fascination, especially for young learners, but problems that seem almost like addiction appear in adults as well. Users can withdraw into a virtual world, where there is a lot of audiovisual simulation and none of the risks or discomforts of being in the non-virtual world. Spending too much time on the computer then leads to other physical health problems, like eyestrain, poor posture and other problems from sitting all day, and inflammation of the wrists.