- there's unit tests - there's integration tests - you can run the code locally and it works - you can send data you control to the system - you can check w

- there's unit tests - there's integration tests - you can run the code locally and it works - you can send data you control to the system - you can check what the system responds with - you can do all of the above in debug mode and breakpoints actually work - the external dependencies are optional or you can control them too on dev environment without too much hassle If I have all of that - maintenance will be easy, SOLID or no SOLID, functional or OOP or whatever.