My guess is that as Bannerlord's codebase grows, so does the time it takes to add new features. The already existing code slows them by being rigid, unreadable, coupled, etc. In software engineering, this is called technical debt. It can be mitigated by following the best practices in software development and having good project management. It is the reason why they say it is too complicated to add new features. They are not lying when they say that. If the code is full of "smells", then every changes takes more time. Remember when they did a refactoring, it was probably to address this issue.