Fr. Peter Scott has pointed out that a positive doubt about the per se validity of the 1968 form of ordination as promulgated does not exist, for the strictly essential part of the form is practically identical to that defined by Pope Pius XII in 1947. It may be invalid in specific cases, he said, owing to defect of intention or poor vernacular translation.
The case of episcopal consecration is slightly more nuanced. In this case Fr. Pierre Marie published a study, which Fr. Scott also referenced in his work, which establishes more or less the same (the per se validity) based on a slightly more complicated argument, namely, that the new rite of consecration is substantially similar to that contained in a third century text of St. Hippolytus called Apostolic Tradition which had been used in Eastern rites, specifically the Coptic and West Syrian.
Both works are an impressive and significant study of the question, and distinguish well something that laymen in particular often forget, the distinction between positive and negative doubt.