Cerebral palsy, spinal cord injuries, stroke, and other neuromuscular disorders may lead to tetraplegia, a condition that results in loss of sensory and motor functions of all four limbs and the torso.

Many approaches are used to help leverage the remaining functional parts of the body for computer interfacing including:

• head tracking
• eye gaze tracking
• speech recognition
• mouth/chin-operated mouse/joysticks
• sip-n-puff
• brain-controlled interfaces
• tongue-operated interfaces

Some limitations of existing approaches:

Eye trackers use eye movements to control the mouse cursor on the computer screen. Unfortunately, they interfere with visual functions because the computers have difficulty determining whether an eye movement was a control input or looking at an object, they require a camera mounted in front of the user, which can block the field of view, and they are susceptible to lighting conditions and need frequent adjustment if the user’s head position is changing.

Voice recognition systems are efficient for typing but are slow and not intuitive for controlling a computer mouse or a powered wheelchair. They are also susceptible to ambient noise, and therefore, not suitable for outdoor environment. 

Head trackers and mouth/chin joysticks require head and neck movements, which do not cover tetraplegics who have limited or no head control, or they cause fatigue in weakened cervical/shoulder muscles, and cannot be used over extended periods. 

Sip-and-puff (SnP) is one of the most popular ATs for driving powered wheelchairs due to its low cost and ease of use, but offers only four commands. It is to slow for computer access and offers limited options. Moreover, the SnP straw and tubing require frequent cleaning or replacement.

EEG-based BCIs are too slow because of the limited EEG bandwidth, and susceptible to motion artifacts and interference. They also have a cumbersome setup procedure and need headcap/gel for good electric contact to the scalp.
The Neuralink has some promise, although not everyone will be willing to have a sizable hole drilled into their skull...

Advantages of tongue-operated interfaces:

Since the tongue is connected to the brain by the well-protected hypoglossal cranial nerve, it generally retains functionality in those with tetraplegia.

The tongue has many inherent capabilities that make it an excellent control modality for those with tetraplegia. The area of the motor cortex dedicated to the tongue and mouth rivals that of the fingers and hand, providing the tongue with sophisticated motor control and manipulation capability. 

Tongue movements are natural, intuitive, and do not need thinking or concentration. Hence, assistive devices that operate based on tongue motion are easy to learn and use. Additionally, the tongue can move both quickly and accurately and since the tongue muscle is similar to the heart muscle it does not fatigue easily. 

Moreover, its position inside the mouth gives the users of tongue-operated devices a certain degree of privacy. 

The dexterous, intuitive, rapid, precise, and tireless motion of the tongue is the ideal interface for those with limited mobility to access computers/smartphones, drive wheelchairs, and interact with their environments.

Summarized from this paper on tongue Interfaces out of Georgia Tech:
Tapping into tongue motion to substitute or augment upper limbs