Replacing nulls or empty strings doesn't always work - what am i doing wrong?

Hi Anonymous - I understand what you are trying to achieve but I am not a fan of Table.ReplaceValue.  I prefer the Table.TransformColumns approach  Check the last paragraph in Section 3. 

The formula would look like this:

= Table.TransformColumns(#"Previous Step", 
 { 
  { "mycolumn", each 
      if _ = "x" then "Yes" 
      else if _ = ""  then "Unknown" 
      else if _ = null then "Unknown" 
      else "No" , type text
  }
 }
)