Hermann Detering is not really a mythicist. That is, his writing, as far as I know, does not explicitly bring into question the historicity of Jesus. He nevertheless merits inclusion in a mythicism-who's-who, however, because his work is essentially a revisiting and a re-formulating of several of the crucial Dutch Radical theses that were influential to the development of modern mythicism, most notably the notion that the provenance, dating, and authorship of the Pauline epistles are essentially unknown—certainly uncertain.
The Dutch Radical school, notorious and influential in a heyday that spanned the late nineteenth and early twentieth centuries, was but a forgotten branch of critical scholarship by the time the "Third Quest" of the historical Jesus was underway. Were it not for Detering's The Fabricated Paul and for Robert Price's expositions and endorsements of these ideas online and in his own books, this rich trove of scholarship would still be neglected or relegated to a mere footnote in the field of biblical studies.
The (not so) Bad
More than just representing the Dutch Radical skepticism regarding the traditional provenance of the Pauline epistles, Detering (as does Price in his The Amazing Colossal Apostle) also adds to this the somewhat radical idea that the apostle Paul was really a kind of Doppelgänger of Simon Magus, a shadowy figure who was vilified by the patristic writers as the source of all manner of heresy within the ancient Church. The idea is not a preposterous one. Indeed, it is a very thought-provoking idea, and he (they) argue(s) for it fairly cogently, but, given the piecemeal and ambiguous state of the sources from which such a hypothesis can be woven, it can only be speculative in the end. I can entertain it for the sake of the kind of thought experiments that are part and parcel of the exploration of Christian origins, but only with the requisite proverbial grain of salt. Perhaps the arid sands of the Levant will yield some surprise codexes in the future that would further support this idea, but as it stands, it has little circumstantial evidence and/or indirect support from extant sources to commend it.
About the only bad thing I found in my reading of Detering's The Fabricated Paul is the weird (in my opinion) section in which he recalls a visit to a library wherein he finally got to read a rare book he had been searching for by Edwin Johnson, Antiqua Mater (1887), which argues for a form of quasi-mythicism. I have read the book myself and found it to be very good. My apprehension is not so much about the content of this book itself but the mystical, almost sycophantic way that he treats the subject of his quest for this rare book. The section reads like a travelogue digression in what is otherwise a fairly rigorous and scholarly study. He sounds like a fanboy there. What makes it even weirder is that, although Antiqua Mater is a fine book, Johnson's subsequent work, particularly his The Pauline Epistles - Re-Studied and Explained (1894), includes the bizarre claim that the historical period that we know as the Middle Ages (700–1400) never really happened, but was instead an invention of Christian writers, which calls to mind the kind of fringe para-historical conspiratorial formulations of someone like Joseph Atwill, and leaves me scratching my head and questioning its author's mental health and motivations. Knowing this about Johnson's work makes this section in Detering's book a bit surreal to me, though it does not detract much from its overall approach and usefulness viz Pauline/Christian origin scholarship.