the only differnece between cages made at home and ones made by companies are that the companies have better tube bending tools to give a bend that is not deformed in the way that would happen with a 'crossbow' tube bender flattens the tubes profile slightly. also access to a good welder and someone used to welding tight turns is deffo an advantage. saying that its not impossible for the DIYer to achieve as strong a cage as the strength comes not only fromthe construction but also the design, if you follow the rules set down by the FIA you shoud be able to produce a good strong safe cage.
just remember that if your not confident with a welder then dont risk your life!