I've inhereted a workbook where the previous owner used a complex {=SUM()}
function to approximate a SUMPRODUCT(). It currently is working fine, but I
wonder if there are any advantages to changing the functions to be actual
SUMPRODUCT. Does using SUM in this way take up more resources/become
unstable, etc?

I'm chasing down some gremlins in my Excel workbooks and this one popped out
at me.

For reference, the current function looks something like this:

{=SUM(($A4='C:\[external-file.xls]sheet1'!$A$9:$A$15000)*(AX$1='C:\[external-file.xls]sheet1'!$C$9:$C$15000)*'C:\[external-file.xls]sheet1'!$F$9:$F$15000))}
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Hmm...they have the Internet on COMPUTERS now!