I would agree with this statement. I think that ethical conduct is the most difficult to define because human beings are poised between equally desirable, yet ultimately incompatible courses of action when it comes to ethical conduct. The mere idea of "ethics" in terms of indicating how one should behave carries with it a multiplicity of contexts and situations that can make it quite difficult to lucidly distinguish what to do. What makes this even more challenging is that ethical conduct has been defined over intellectual history in a variety of different ways. For example, the Classical notion of ethical conduct is in stark contrast to a more modern and consequentialist base of inquiry. Part of the reason why ethical conduct and ethics remain difficult to define is that one can only speak from their own experience. There might be a debate as to which set of standards or conditions might be better than another or which criteria is more valid in order to define ethical conduct, but in the end, individuals have to decide for themselves. Within such autonomy is little other than one's beliefs, and this sense of forlorn makes absolute and totalizing definition very challenging.
In addition to the above answer, ethics has be defined by various persons according to their understanding and experience in life. Some definitions are even influenced by the culture and social norms of the environment where we live.
In trying to define ethics a scholar once said it is like defining philosophy; there is no one definition.
Ethics or ethical standards are defined by the society, culture, norms, lifestyle and background of various people in varying aspects of life. What seems ethical in one society or to one person might not be ethical to others.
For example, a man might feel free in the west to allow his wife wear whatever she wishes but that same dressing in the east might be translated into an unethical one.
Noteworthy is the fact that universally, there are ethical standards that guides a group of people in a society, but individuals derive their standards from the generally accepted norms or ethical standards of that society.
Therefore, there is no gain saying that defining ethics is the most difficult concept to define as philosophy is. Defining ethics is philosophy itself.