It can lead to tightly coupled and hard-to-maintain code.
When module boundaries are not enforced, code from one module can access internal classes from another, leading to dependencies that are hard to track and manage. In an e-commerce application, if the 'orders' module directly accesses the 'database' module's internals, any change in the database module could break the orders module, making maintenance cumbersome.
Additional Notes
What happens if module boundaries are not enforced?