The issues with base64 or similar things are:
- you can not diff on it naive and natively
- it is less compactable (“zip”, double delta compression, …)
- you need
base64encoding/decoding as a extra step when transport as json is given - not readable in native format
This also applies to binary formats or other codings.
JSON has in addition the advantage that basically all modern databases and other sotftware supports it natively.
As so often in software there are pros and cons and it depends on your needs if the approach fits. Of course there are other options and pathes one can choose for implementation or architecture.