[AlexanderNull]

Technical debt. Some places let this stuff mount up under constant pressure from product or management to always deliver features as quickly as possible. This makes it more difficult to track down bugs or add new features as this mounts. Most often it severely impedes on the ability for low-mid level engineers to write good quality code as the complexity of refactoring the existing issues surpasses their skill level.

I guess the big pain point then is actually having management that doesn't support tech debt cleanup at any appropriate level. Without support, budgeting you'll end up at a point where even as a skilled developer your choices are to a) say to hell with your managers guidance and blow the whole iteration refactoring just so you can even start on the right footing with a new module, or b) write code that isn't as reusable, maintainable, or performant.