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I guess you mean typedefs and not separate classes for Money, TaxPercentage etc.

I recently came across a codebase that implement type classes for almost all types of things it handles and found that to be overkill. Almost always, name and age can be represented by a string and an integer respectively; separate Name and Age classes just reduce the readability in your code.

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In a language that supports it reasonably, the advantage of genuinely separate types is that the language will prevent you from doing stupid things like accidentally multiplying to monetary amounts or adding a percentage to a monetary amount.

Reasonable support here mostly means that you can implement operator overloads, so that you can define e.g. a Money type that can be added but not multiplied.

Money handling may not actually be such a good example for this, because you should really have proper unit testing there anyway. I did work on a codebase that had types for SI units, and it was quite a nice experience.

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