Self-modifying code is difficult to understand because by definition the code you are looking at might change without you observing the change. So the source code being the 'source of truth' that we are all accustomed to is not true because another line might modify the one your looking at. This problem is similar to heavy use of monkey patching or OOP patterns where "everything happens somewhere else". I'm not arguing for or against self-modifying code, but it is important to note that there is a cost associated with it.
"In computer science, action at a distance is an anti-pattern (a recognized common error) in which behavior in one part of a program varies wildly based on difficult or impossible to identify operations in another part of the program."
