JSON doesn't fail for very large values because they are sent over the wire as strings. Only parsers may fail if they or their backing language doesn't account for BigInts or floats larger than f64, but these problems exist when parsing any string to a number.

And indeed applies to CSV as well: it's just strings at the end of the day, its up to the parser to make sense of it into the data types one wants. There is nothing inherently stopping you from parsing a JSON string into a uint64: I've done so plenty!