it's good that migration agents refund you if you fail. But still the lodgement fee is not refundable. So it's the same thing as lodging the application yourself. Who knows the migration agent will run away and disappear with your money

. Now, that would be a double disaster.
Let me give you more info why using agents isn't gonna help you at all. Every year IMMI updates their Priority List of the Professions that Australia needs. Each profession has corresponding points in it. Let say if OZ needs more Nurses next year than Electrical Engineers, then the Nursing profession has bigger points than EE profession. If you are an EE and say you lodge your application, spending money for a migration agent is not gonna help you get more points compared to the Nurses. Remember, this is just an example. All the detailed info is at IMMI website. That's why you must READ and UNDERSTAND what is printed at the IMMI website.
There are many more factors that affects your application namely; your age, your work experience, English language, where you want to live in OZ, etc. These things has corresponding points. By just following the IMMI steps/procedure, then you are halfway there.
Your documents will not be rejected for as long as you follow the IMMI procedures and that your documents are not fake. Again READ and UNDERSTAND what is on the IMMI website.
cheers,
Bisaya Dako