power consuption is a massive part of the design phase. if you want a robot to do anything it needs power. and power adds wieght which needs more power...... et al
reliablity is high, probably 2nd/3rd. covers a lot though, changes in voltage due to battery going flat, vibration from itself, minor robot enviroment interactions that didn't go to plan...
cost is the biggest limiting factor. the pervalence of robots is limited by the cost of building them. if two motors cost 50 quid, and a sharp ir 10, the robots easily over 100 quid.