One thing that surprises me is that the --warn-uninitialized flag does not issue a warning when ${X} is used and the variable X has been explicitly unset using unset(X) or set(X). For example:
if(NOT DEFINED X)
message("'X' is undefined")
message("X=${X}") # warning: uninitialized variable 'X'
endif()
unset(X) # or set(X)
if(NOT DEFINED X)
message("'X' is undefined")
message("X=${X}") # no warning
endif()
The relevant output is:
'X' is undefined
CMake Warning (dev) at example.cmake:5 (message):
uninitialized variable 'X'
This warning is for project developers. Use -Wno-dev to suppress it.
X=
'X' is undefined
X=
It appears that the current behavior treats unset(X) (or set(X)) as if it initializes the variable X, even though it is actually undefined. 
I find this behavior unintuitive and unhelpful. It also makes --warn-uninitialized inconsistent with if(DEFINED <variable>), as demonstrated above.
If the goal is to initialize a variable with an empty string, the appropriate and intuitive command should be set(<variable> ""). Moreover, unset(X) is not equivalent to set(<variable> ""), because unsetting a variable may expose a cache variable with the same name. So it remains unclear to me how the command unset can be logically associated with the concept of initialization.