The article addresses this. The open source implementations are not complete so there are compliant files they cannot render. Also they have rendering bugs which mean some content does not render properly, and worse some files they generate render fine now, but are not compliant and if the code is fixed those portions of the files may stop rendering correctly. Some of these issues only manifest at particular zoom levels, so you are never be sure what you are seeing is correct. This us all part of the problem with the spec being so complex.