I just added a Repeater Field with a few sub fields. Just saw that HTML code inside “field instructions” causes some errors. Sounds logically on the one hand. On the other hand you may want to instruct the authors to use specific HTML snippets, like: “Please insert the <iframe … > widget code”.
Drag’n’drop reorder of all sibling custom fields won’t work afterwards. The wordpress post editor stops loading after the new custom field.