The extent of integration imo depends on what kind of system you're running. For some systems, especially minimalist ones, I agree the BSDs tend to be more integrated, though they do still heavily depend on third-party stuff (the compiler, whether LLVM or GCC, being a big one).
However, once you start adding some additional packages, Debian tends to do the integration-testing more pervasively across the entire userland, "Debianizing" packages so that they all store things in the standard locations, are started/stopped using the same methods, follow the same configuration policies, etc. Once you get out into ports, the BSDs tend to give you more of an as-is set of 3rd-party packages, and are pretty lax about integration testing.
> (the compiler, whether LLVM or GCC, being a big one).
Although I think the effort is misguided, they are working pretty hard on that one. There is some considerable effort going on to get pcc as the default compiler for the BSDs, specifically so that they don't have to rely on another upstream (and so they don't have to use a GPL'd compiler).
However, once you start considering obviously 3rd party packages, then not even windows could be considered a single project piece of software.
However, once you start adding some additional packages, Debian tends to do the integration-testing more pervasively across the entire userland, "Debianizing" packages so that they all store things in the standard locations, are started/stopped using the same methods, follow the same configuration policies, etc. Once you get out into ports, the BSDs tend to give you more of an as-is set of 3rd-party packages, and are pretty lax about integration testing.