The correct comparison of the vulnerability of the heads of executive departments versus those of regulatory agencies is:
The heads of the former can be fired at the will of the president; while the heads of the latter can’t be fired during their terms except for extreme misconduct.
This reflects the general principle that heads of executive departments serve at the pleasure of the president and can be dismissed at any time, while leaders of independent regulatory agencies typically have fixed terms and can only be removed for cause, such as malfeasance or misconduct, providing them with more job security.