The brittleness comes from the fact that the number of implied decimal digits per currency isn't always well-defined across all stakeholders and systems.
replyIf you're only working in a single currency, there's usually no issue.
As a general rule, you always include the currency code (EUR, SEK, USD etc.) and if possible also the amount of decimals, when using minor units.
replyCurrency codes can be found in ISO 4217.
Yes, definitely always include the number of digits, but at your system boundary you still have to pray that whoever you're working with isn't silently dropping that number and re-deriving it from their own, almost-4217-compliant currency database.
replyRedundancy can be great, but it's not a panacea, since it's not guaranteed to be used in an optimal way.
ISO 4217 also defines the number of significant digits after the decimal separator if Wikipedia is to be believed.
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