Con: If the executive branch of the government has the ability to make these weapons, the legislative branch will no longer pose a threat to them. Legally, they'll be forbidden, but practically speaking, they will not be prevented. Laws don't prevent people from behaving badly, nor do they guarantee that bad behavior will be punished, they just specify consequences and define the bad behavior. The consequences are contingent upon whether the person is caught and whether the authorities have enough power to dole out a punishment. In the event that the lawbreaker gains so much power that the authorities can't stop them, the threat of punishment is N/A. A law can't solve the checks and balances issue.