A maximum of 100 entries can be embedded within the JSON RTE.
A maximum of 15 content types can be selected as reference for a single JSON RTE.
The maximum number of instances allowed for JSON RTEs marked as "Multiple" is 100.
During migration, both the HTML-based and JSON RTEs should be at the same level of field depth to migrate content successfully.
If the JSON RTE is marked as "Multiple" and HTML RTE is of "Single" type, then you won't be able to migrate content.
During migration, the HTML RTE and JSON RTE should allow embedded entries from the same set of referenced content types. If the referenced content types differ, you cannot migrate content between the RTEs.