Custom Drupal codebases (or any codebases really) can be difficult to maintain. New developers to the system need to familiarise themselves with how the code works and what the system is doing before they can make any contributions.
What makes things more difficult is when a site with lots of custom code has modifications that adapt the functionality to specific users.
I will occasionally come across sites that provide some sort of service, but have made modifications to their code that changes things for a particular user (or set of users). This is often to appease the largest user base on the system that wants things done in a certain way, but the site is unwilling to change things for all users.
Read more here: https://www.hashbangcode.com/article/drupal-9-using-custom-hooks-and-events-custom-code