TAG_String has a UTF-8 payload, which makes it more expensive to work with. Also, skins can contain bytes which are not valid UTF-8 characters and will therefore be treated as corrupted by external tools.
Additionally a TAG_String can only hold 32767 bytes, which might become a problem in the future.
A TAG_ByteArray can hold up to 2GB of data, and there is no character encoding restrictions on it.