In my research I found something called the Open XML 2.5 Productivity Tool for Microsoft Office and it's proving invaluable in figuring out what is making Excel unhappy. For example, if it doesn't seem to like ampersands in property values. (i.e. setting CompanyName to "My Company & Son" makes Excel complain when opening the file).
It also appears to dislike other characters such as the one referenced below (the fun continues):
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Open XML SDK 2.5 Productivity Tool for Microsoft Office
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Cannot open the file: Part /xl/sharedStrings.xml: '', hexadecimal value 0x18, is an invalid character. Line 1373, position 2368.
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