According to the Code of Conduct we do not have a legal right to provide you with any guarantees of success. 
The decision as whether to grant visa or not is made by the Department of Immigration and not us.

Every case is individual and probability of success is predictable only throughout the process of application preparation. The closer we move to the application submission date the more chances are there to predict the result. 
For example, skilled visa success in most cases is quite predictable taking the documents were correctly prepared. There is almost a 100 percent chance to predict it right as there is no broad interpretation of the legal criteria. While for example partner visa has a requirement of a genuine relationship, which includes a number of aspects which do not have a straight forward interpretation. 
That is why there is a great difficulty in any predictions especially at the stage when an agent has not seen any documents yet.

What we can guarantee is honesty in assessment of your chances to immigrate; we can guarantee our competence in dealing with migration matters of our clients; our dedication to you and our diligent work.

Be careful with those who give you so called guarantees and refer to their personal connections with the Department of Immigration. They dissemble.