Calling cmd.exe in Windows follows a very different pattern from Vim.
The primary difference is that Vim does a nested call to cmd.exe, e.g.
the following call in Vim
system('echo a 2>&1')
spawns the following processes
"C:\Program Files (x86)\Vim\vim80\vimrun" -s C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
^>C:\Users\dummy\AppData\Local\Temp\VIoC169.tmp 2^>^&1)
C:\Windows\system32\cmd.exe /c C:\Windows\system32\cmd.exe /c (echo a 2^>^&1
^>C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2^>^&1)
C:\Windows\system32\cmd.exe /c (echo a 2>&1
>C:\Users\dummy\AppData\Local\Temp\VIo3C6C.tmp 2>&1)
The escaping with ^ is needed because cmd.exe calls itself and needs to
preserve the special metacharacters for the last call. However in nvim
no nested call is made, system('') spawns a single cmd.exe process.
Setting shellxescape to "" disables escaping with ^.
The previous default for shellxquote=( wrapped any command in
parenthesis, in Vim this is more meaningful due to the use of tempfiles
to store the output and redirection (also see &shellquote). There is
a slight benefit in having the default be empty because some expressions
that run in console will not run within parens e.g. due to unbalanced
double quotes
system('echo "a b')