The disadvantages of lower orbits satellites are that you have got very short link duration, because the satellite is visible to you, only for a short while, and therefore if one uses satellites for mobile communication, more hand over will be needed of the order of 4 to 5 times more hand over will be needed compared to the terrestrial mobile communication system.
Satellites in lower orbits satellites will have a smaller coverage area, and, since they are in low orbits satellites and you have to switch over from one satellite to other for connectivity, the control systems are quite complex.