Workaround: use a custom RichTextEditorRibbonBar

Public Class CustomRichTextEditorRibbonBar
Inherits RichTextEditorRibbonBar

    Dim headerText As String
    Protected Overrides Function GetInsertTableItems() As RadItemCollection
        Dim collection As RadItemCollection = MyBase.GetInsertTableItems()
        Dim headerItem As RadMenuInsertTableItem = TryCast(collection(0), RadMenuInsertTableItem)
        headerText = "My Header Text"
        Dim fi As FieldInfo = GetType(RadMenuInsertTableItem).GetField("header", BindingFlags.NonPublic Or BindingFlags.Instance)
        Dim header As LightVisualElement = fi.GetValue(headerItem)
        header.Text = headerText
        AddHandler header.TextChanged, AddressOf header_TextChanged
        Return collection
    End Function

    Private Sub header_TextChanged(sender As Object, e As EventArgs)
        Dim lve As LightVisualElement = TryCast(sender, LightVisualElement)
        If lve.Text = "Insert Table" Then
            lve.Text = headerText
        End If
    End Sub
End Class

Please refer to the attached screenshot. It is not possible to localize the marked label.

Workaround:

 Sub New()
     InitializeComponent() 
     Me.RadRichTextEditor1.RichTextBoxElement.InsertHyperlinkDialog = New CustomInsertHyperlinkDialog()
 End Sub

 Public Class CustomInsertHyperlinkDialog
     Inherits InsertHyperlinkDialog
     Protected Overrides Sub OnLoad(e As EventArgs)
         MyBase.OnLoad(e)
         Me.Controls("radLabel4").Text = "My address"
     End Sub
 End Class

Add support for shapes, and especially for shapes with textual content. 
In OOXML, shapes are represented by the wps:wsp element, and shapes with textual content by <wps:txbx>, <w:txbxContent>.
Such shape can be added to a Word document using the Insert -> Text -> Text Box -> Draw Text Box, or through a shape's context menu -> Add Text.

Note: Do not confuse with Text/Rich Text content controls (http://feedback.telerik.com/Project/154/Feedback/Details/156478 )

To reproduce: please refer to the attached sample project.

Workaround: export RadRichTextEditor's content to a .doc file. Then, use the RadWordsProcessing library to import the .doc file and export it as a pdf:

Telerik.WinForms.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider provider = 
    new Telerik.WinForms.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
string fileName = @"..\..\exported.doc";
string pdfFileName = @"..\..\exported.pdf";
Stream s = new FileStream(fileName,FileMode.Create, FileAccess.Write);
provider.Export(document, s);
s.Close();
s.Dispose();

Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider provider2 = 
    new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();
using (Stream input = File.OpenRead(fileName))
{
    Telerik.Windows.Documents.Flow.Model.RadFlowDocument document2 = provider2.Import(input);
    Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider provider3 = 
        new Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider();
    using (Stream output = File.OpenWrite(pdfFileName))
    { 
        provider3.Export(document2, output);
    }
}

System.Diagnostics.Process.Start(pdfFileName);       

To reproduce:
1. Insert a table
2. Select all cells in it
3. Press the Backspace key => the table is removed but the table adorner is still visible

The adorner disappears if clicked

To reproduce:
- Add some misspelled words.
- Enable the spell check - the layout is updated and the words are underlined.
- Disable the spell check - the lines are not removed until one clicks in the RichTextEditor.

Workaround:
 radRichTextEditor1.IsSpellCheckingEnabled = !radRichTextEditor1.IsSpellCheckingEnabled;
 radRichTextEditor1.Document.LayoutMode = DocumentLayoutMode.Paged;
 radRichTextEditor1.Document.LayoutMode = DocumentLayoutMode.Flow;

Workaround:
string styleSuffix = "_1";
foreach (var importedStyle in rtfDoc.StyleRepository)
{
        importedStyle.Name = string.Concat(importedStyle.Name, styleSuffix);
}

Implement commands and UI for producing labels using Mail Merge. 

Behind the scenes this is implemented as Table with the proper number of table columns and rows, and eventually (depending on the setup) NEXT field (https://support.office.com/en-us/article/Field-codes-Next-field-3862fad6-0297-411a-a4e7-6ff5bcf178fd?ui=en-US&rs=en-US&ad=US) - so this will eventually require implementing NEXT field.

Note: In MS Word, this is in Mailings -> Start Mail Merge -> Labels...

Merge all assemblies exept Telerik.WinControls.UI.Design.dll.
Use the merged assembly in an application with RichTextEditorRibbonBar.
The icons are missing. 

When the specified length of the stream is larger than the actual one, Adobe throws errors and removes the image.

Workaround:

Set the format provider settings:

PdfFormatProvider provider = new PdfFormatProvider();
PdfExportSettings exportSettings = new PdfExportSettings();
exportSettings.ContentsDeflaterCompressionLevel = 9;
exportSettings.ImagesDeflaterCompressionLevel = 9;
provider.ExportSettings = exportSettings;

Workaround:
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        this.radRichTextEditor1.IsSpellCheckingEnabled = true;

        this.radRichTextEditor1.Insert("SOooome wrrrrong wooooooooords.");

        this.radRichTextEditor1.CommandExecuted += radRichTextEditor1_CommandExecuted;
    }

    bool shouldProcess = true;
    private void radRichTextEditor1_CommandExecuted(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutedEventArgs e)
    {
        if (e.Command is DeleteCommand)
        {
            if (shouldProcess)
            {
                shouldProcess = false;
                RichTextEditorInputBehavior behavior = this.radRichTextEditor1.InputHandler;
                behavior.ProcessKeyDown(new KeyEventArgs(Keys.Back));    
            }

            shouldProcess = true;
        }
    }
}

To reproduce:

radRichTextEditor.Document = new RadDocument();
radRichTextEditor.Document.MeasureAndArrangeInDefaultSize();
radRichTextEditor.UpdateEditorLayout();
radRichTextEditor.Document.CaretPosition.MoveToLastPositionInDocument();

var p = new Paragraph();
p.Inlines.Add(new Span("Cell"));
var cell = new TableCell();
cell.Blocks.Add(p);
var row = new TableRow();
row.Cells.Add(cell);
var table = new Table();
table.Rows.Add(row);
var section = new Section();
section.Blocks.Add(table);
radRichTextEditor.Document.Sections.Add(section);
radRichTextEditor.Document.CaretPosition.MoveToLastPositionInDocument();

var provider = new RtfFormatProvider();
var txt = provider.Export(radRichTextEditor.Document);
Clipboard.SetText(txt, TextDataFormat.Rtf);

Workaround: measure the document after the section is added:
radRichTextEditor.Document.Sections.Add(section);
radRichTextEditor.Document.MeasureAndArrangeInDefaultSize();
radRichTextEditor.UpdateEditorLayout();

To reproduce:
- insert a fragmet from another RichtextEditor like this:
private void Button_Click(object sender, RoutedEventArgs e)
{
radRichTextBox.Document.Sections.Clear();
radRichTextBox.Document.Sections.Add(new Section());

radRichTextBox.Document.CaretPosition.MoveToLastPositionInDocument(); 
radRichTextBox.DocumentEditor.InsertParagraph();
radRichTextBox.DocumentEditor.InsertFragment(new DocumentFragment(radRichTextEditor1.Document));
radRichTextBox.DocumentEditor.InsertParagraph();
}

Workaround:
There is no point of resetting the section in such way. Nevertheless, you can avoid the exception by using one of the following methods:

radRichTextBox.Document.MeasureAndArrangeInDefaultSize();
radRichTextBox.UpdateEditorLayout();

Local properties are not exported to HTML when style is applied over document element (paragraphs, spans, tables) in case the HtmlExportSettings.StyleExportMode is set to Inline.

To reproduce:
- Add two hyperlinks with differents font and font weights.
- Export them with the following code:
private void button1_Click(object sender, EventArgs e)
{
    HtmlFormatProvider html_provider = default(HtmlFormatProvider);
    string htmlBody = null;
    html_provider = new HtmlFormatProvider();
    html_provider.ExportSettings.StylesExportMode = StylesExportMode.Inline;
    html_provider.ExportSettings.StyleRepositoryExportMode = StyleRepositoryExportMode.ExportStylesAsCssClasses;
    htmlBody = html_provider.Export(radRichTextEditor1.Document);
    File.WriteAllText("test.html", htmlBody);
}
- Open the file and you will notice that the custom styles are not exported (the links are having the same style).

Workaround: 
html_provider.ExportSettings.StylesExportMode = StylesExportMode.Classes;