You have to write applications in a different way: one which makes them testable. You'd be surprised how difficult this is at first.
Some people find the concept of thinking about what they're going to write before they write it too hard. Concepts such as mocking can be difficult for some too. TDD in legacy apps can be very difficult if they weren't designed for testing. TDD around frameworks that are not TDD friendly can also be a struggle.
TDD is a skill so junior devs may struggle at first (mainly because they haven't been taught to work this way).
Overall though the cons become solved as people become skilled and you end up abstracting away the 'smelly' code and have a more stable system.
