The press needs to be trustworthy in the eyes of the public. That is its main capital, but too often its actions further undermine its credibility. Therefore, the fact that citizens generally believe that journalists would get any story at any cost needs to be an important consideration. Exposes that rely on questionable methods to get information can further diminish the legitimacy and public standing of the reporting and the journalists. Ethical issues are not limited to methods.
Corruption is also another important ethical issue in investigative journalism.
Corruption includes a variety of practices, ranging from journalists who accept bribes, or quash exposes, or pay sources for information. The harm to private citizens that might result from what's reported also needs to be considered. Issues of privacy usually come to the forefront, as investigative journalism often walks a fine line between the right to privacy and the public's right to know. It is usually assumed that privacy applies differently to public figures than to average citizens. There are no easy, ready-made answers to ethical issues.