Honestly, I like to work on such systems because:

- there is so much stuff to improve. There’s nothing better than the feeling of improving things with code (or by removing it)

- it’s back to school again and everything goes. You can implement features in any way you want because the constraints the system imposes. Granted, sometimes it’s painful to add functionality

- there’s usually no room to subjective topics like clean code and architecture. The most important thing with these systems is correctness (and this is usually an objective topic)

- nobody can blame you for something that doesn’t work. It’s always the fault of the legacy system

I wouldn’t recommend working on such systems to junior engineers, though.

I don’t really like to work on “perfect” codebases where everyone follows the same pattern, with linters, where if something breaks is because of your shitty code (because the codebase is “clean”). It’s very frustrating and limiting.