Limitations of mobile computing

Insufficient bandwidth
If the user needs access to a network such as the internet on the go, they must resort to slow wireless WAN systems primarily intended for telephone use. Higher speed wireless LANs are only available in specific sites.
Security standards
When working mobile one is dependent on public networks, requiring careful use of VPNs.
Power consumption
Since the grid can not be used, mobile computers must rely entirely on battery power. Combined with compact size, this means unusually expensive batteries must be used
Transmission interferences
Weather and terrain problems as well as distance-limited connection exist with some technologies. Reception in tunnels and some buildings is poor.
Potential health hazards
Potential health damage from cellular radio frequency emission is not known yet. However, more car accidents are related to drivers who were talking through a mobile device. Also, cell phones may interfere with sensitive medical devices.
Human interface with device
As HMDs are still uncommon, screens are often too small. Keyboards are impractical, especially one-handed, and alternate methods such as speech or handwriting recognition require training.
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source: Wikipedia