CMake OS name flags like APPLE and UNIX are terse and are frequently used. A possible downside is their have broad, overlapping meanings.

In contrast, CMAKE_SYSTEM_NAME has more fine-grained values.

However, it is often more convenient (if using care) to use terse variables that are not as specific:

if(APPLE)
  # macOS
elseif(BSD)
  # FreeBSD, NetBSD, OpenBSD, etc.
elseif(LINUX)
  # Linux
elseif(UNIX)
  # Linux, BSD, etc. (including macOS if not trapped above)
elseif(WIN32)
  # Windows
else()
  message(FATAL_ERROR "Unsupported system: ${CMAKE_SYSTEM_NAME}")
endif()

There is not one “right” or “wrong” way.