Some sedes believe J23 was a true pope. Only a very few fringe people who are almost always home aloners believe Pope Pius XII was a manifest heretic. There is no real evidence of it. The arguments I have heard have to do with the fact that modernists infiltrated the hierarchy during his reign. But even the Dimonds believe he was a true pope and if anyone should think he was a false pope I would think it would be anti-BOD folks who are not happy about the protocol sent to Cushing in 1949.
I have met some folks who cringe upon the mention of Pope Pius XII because they seem to be anti-Vatican I Old Catholics. They are heretics in their own way as many of them now accept ɧoɱosɛҳųαƖ unions, women priests, and have joined with Episcopalians. Not surprisingly, these Old Catholics often celebrate services in the vernacular, although you will find some that celebrate the Latin Mass. Without a Pope, they have become their own pope. They tend to dislike Pius XII especially because:
1) As a canon lawyer, the future Pope Pius XII worked on the Code of Canon Law of 1917, which changed canon law to be in conformance with Vatican I.
2) As Pope, Pius XII changed the Latin Missal several times by adding Saints to the Church Calendar and by changing the Services during Holy Week, notably the Good Friday Services.
3) As Pope, Pius XII allowed the Germans to celebrate the Mass in German during World War II. During that same war, the British also celebrated the Mass in the vernacular, notably the Jesuits, especially the heretic Teilhard de Chardin.
4) As Pope, Pius XII issued the Encyclical
Mystici Corporus, which established a Diocesan Liturgical Commission in every diocese. These diocesan liturgical commission encouraged the use of the vernacular in the Mass, and ultimately brought on Vatican II.
and finally
5) Didn't Pope Pius XII shortly before his death start the work to bring on Vatican II to complete the work of Vatican I?