One limitation of JSON is its limited set of types. For example, for decimal numbers one has to resort to stringly typing representation because DB connection libraries assume that JSON numbers are floating point. Note that JSON numbers are just sequences of digits, nothing more. There is no attached precision semantic.
Another example is UUIDs. Instead of transferring 16 bytes, the libraries deal with wasteful string representation. I'm sure you can bring another examples.
Nonetheless, for majority of data JSON as DB output format is alright.
Yeah, json is annoying because of the limited types, but postgres arrays/rows are annoying because of the lack of sum/union types (if your UI has a heterogenous list of elements).
Another example is UUIDs. Instead of transferring 16 bytes, the libraries deal with wasteful string representation. I'm sure you can bring another examples.
Nonetheless, for majority of data JSON as DB output format is alright.