Least likely to be an issue? I really do think their #5, "Cost of immigration reform" is the least of it. Economists should be able to easily make a case for the economic boon legalized immigration would bring. Many think tanks are already issuing reports touting the economic benefits of reform. Also, the legalisation could build in eligibility dates into the future for benefits eligiblity. Under current law, Green Card holders risk deportation if they access welfare programs.
Most likley to be an issue, #4, "Keeping the support of labor and business." Politico is already reporting some business are holding their fire until the legislation is revealed.