The correct comparison of the vulnerability of the heads of executive departments versus those of regulatory agencies is as follows:
The heads of executive departments can be fired at the will of the president, while the heads of regulatory agencies often have more job security and can only be removed for specific reasons, such as extreme misconduct.
This means that heads of executive departments are generally more vulnerable to dismissal by the president compared to heads of regulatory agencies, who typically have protections that allow them to serve their terms unless there is significant cause to remove them.