When a document has a list in it and the element containing it is cloned into another document, there is a key not found exception if you try to export the second document.

            RadFlowDocument document = new RadFlowDocument();
            if (tableDocument.Sections.First().Blocks.First() is Table workSheetTable)
            {
                Table clonedTable = workSheetTable.Clone(document);
                var newSection = document.Sections.AddSection();
                newSection.Blocks.Add(clonedTable);
            }

            byte[] pdfBytes = pdfProvider.Export(document);
            byte[] docBytes = docxProvider.Export(document);

Generate a document with the following approach: https://docs.telerik.com/devtools/document-processing/knowledge-base/populate-table-data-mail-merge

Before calling MailMerge, add two document variables with the editor: https://docs.telerik.com/devtools/document-processing/libraries/radwordsprocessing/concepts/fields/document-variables 

When a style doesn't have explicitly defined font, the font from the default character properties (\defchp) is used when present. Instead, the font used for the style should be the default font for the document, defined with \deffN tag.
The same issue occur when the doesn't contain \defchp at all. In this case, the font for the style is not imported, but instead the defined with  \deffN tag should be used.

The construction is not common, and MS Word for example doesn't produce such documents.

When a paragraph has defined tab stops in its style and has the same tab stops locally cleared, the tab stops are still respected when exporting to PDF. Instead, cleared tab stops should not be respected.

Workaround:
Copy the default styles after the Mail Merge

RadFlowDocument merged = sourceDocument.MailMerge(mailMergeSource);
merged.DefaultStyle.ParagraphProperties.CopyPropertiesFrom(sourceDocument.DefaultStyle.ParagraphProperties);
merged.DefaultStyle.CharacterProperties.CopyPropertiesFrom(sourceDocument.DefaultStyle.CharacterProperties);

If the value from the source contains new lines, they should be inserted in the resulting documents as new paragraphs.

The <font> tag has a 'size' attribute that can be set to a number from 1 to 17. This is currently not supported and is just ignored.

CSS style property with a numeric value will not be imported from HTML when the unit type is not specified. Example: .td{padding-bottom: 200;}

This is invalid according to the HTML spec (value is optional only after 0), though web browsers and MS Word open such HTML's correctly.

In specific situations, a paragraph in a table cell which is last on a document page is not exported to the PDF document.
Possible workaround is adding a new paragraph to the last table cell before exporting the document.
For instance:
BlockCollection footerContent = this.document.Sections.First().Footers.Default.Blocks;
Table footerTable = footerContent.First() as Table;
footerTable.Rows.Last().Cells.First().Blocks.AddParagraph();