Recall that Giuga's conjecture (1950), still widely open, asserts: let $n$ be a positive integer, if $1+\sum_{k=1}^{n-1}k^{n-1} \equiv 0\pmod{n}$ then $n$ is prime.
In light of the question and answers showing Gauss' generalization of Wilson's theorem is implied by group-theoretical results (classification of $n$ for which the units mod $n$ are cyclic), I'd like to ask:
are there group-theoretical results that make it difficult to believe that Giuga's conjecture is true?
Partial answers or numerical considerations welcome.