The migration process is a legal process. Many migration agents are not lawyers, and for this reason are not familiar with the law outside the area of migration. This can be significant to your application if the agent does not have the depth of experience or expertise in law generally to be able to advise you in relation to all areas of your visa application.
An agent who is not a lawyer may not understand the full significance or interrelationship of legal principles underpinning the administration of migration law.
You should always check whether a migration agent is also a lawyer before selecting that agent to work with you on your visa application.