The basic difficulty arises because the external attributes such as maintainability are not solely dependent on a small number of internal product attributes; there are more factors that can affect this.
While the complexity of a system may influence the said system's maintainability, other issues such as the choice of variable names, system documentation and, particularly, the skills of the people doing the maintenance have such a large effect on the process that they may underplay any maintainability differences arising from different levels of complexity in the system.
It is important to know that the submission above does not contradict experiments where a relationship between maintainability and complexity is established – however, there is not enough evidence at the moment to generalize this.
Essentially, the relationship between internal product attributes such as cyclomatic complexity and external attributes such as maintainability is one that is difficult to validate because there are so many other factors (both tangible and non-tangible) that affect the relationship.
If you found this answer helpful, please upvote and share with other students in your network.