Irrelevant.
I have read some critics of Yockey and Imperium term Yockey a plagiarist since much of Imperium has been lifted from other authors, Spengler being just the most obvious.
Now, I think this is all irrelevant. First, Imperium (and all of Yockey's writings) is a polemic work, a call for action, not a scholarly treatise that must come with academic-style footnotes and references.
Second, when did Yockey ever deny that he's borrowed from other people; it's obvious for example the debt he has for Spengler's work; indeed, Yockey praises Spengler as "The Philosopher" (of History).
And, in the Foreword to Imperium, Yockey openly writes:
There is nothing original in the content of this book, the book itself only is original.
That's hardly the statement of a "plagiarist" attempting to take credit for the work of others.
Third, even if Yockey was a plagiarist, does that invalidate his message? Does it invalidate the synthesis of others' ideas that he puts forth? Hardly.
These criticisms of Yockey are completely irrelevant to his work.