The Kirby COTU stories-- some or all of which may've started from full scripts by other writers, though Mark Evanier asserts that Kirby usually rewrote any scripts he was given-- resemble the kind of stuff Kirby did in his mid-to-late 1970s Marvel works. The narratives usually begin with the Challengers being summoned to investigate or eludicate some mystery, usually one with strong SF-aspects, and thereafter the heroes fights aliens or robots or whatever in fast-action stories with no great emotional resonance.Expanding on this somewhat:
Starting in 1957, Jack Kirby worked on the feature CHALLENGERS OF THE UNKNOWN over the course of four appearances in SHOWCASE (issues #6, 7, 11, and 12) and eight issues of the heroes' own feature. In all of these issues the Challengers shared no space with any co-features, though on occasion a given issue might have two short COTU stories rather than a full-length tale. In the recent b&w reprint of the early issues credit for writing issues is given either to Dave Wood or to Kirby himself, though there's no preface to explain by what process DC Comics allocated these credits.
COTU can be fairly regarded as Kirby's first solid-selling series following the end of his business partnership with Joe Simon in 1955. But as I noted above, most of the COTU stories are pretty formulaic despite the Kirby touch. In the shorter stories, Kirby's works seems constrained by what may have been attempts to fit a conservative DC "house style" (which Gil Kane asserted to be most influenced by the example of comic-strip artist Dan Barry). The full-length stories-- in particular a wild time-travel story in COTU #4-- often show Kirby returning to a style of "epic" storytelling that recalls his most successful Golden Age work. However, the plotting of these stories lacks the relatively-strong characterization seen in the Golden Age work Kirby produced in partnership with Simon. The four heroes-- Ace, Prof, Rocky, and Red-- are largely interchangeable aside from their particular talents (diving, piloting, etc.) Often the plots, while reasonably efficient for this type of helter-skelter adventure, have a "fly-by-the-seat-of-one's-pants" quality; one that reminds me of the pace Kirby employed on most if not all of his solo works of the 1970s and thereafter. The dialogue is never as eccentric as one finds in the 1970s solo works, but one presumes that credited co-writers like Wood were responsible in some instances, while in others Kirby himself kept to the then-current writing-standards of his employers.
To be sure, about a year after the debut of COTU Kirby collaborated with both Dave Wood and his brother Dick on the comic strip SKY MASTERS, which prefigures a return to more nuanced characterizations than one sees in COTU; again, more in tune with the Simon/Kirby level of storytelling. The SKY MASTERS strips I've seen have a rather Caniff-like quality in respect to characterization; nevertheless, the short-lived strip (58-61) is at best an interesting experiment.
One of the oddest aspects of the COTU is that while Kirby was working on the title, the characterization of the main heroes had none of the individual development of even some of the one-shot characters Kirby portrayed in his anthology monster-stories in the adjoining years, for such "Marvel" magazines as TALES OF SUSPENSE and TALES TO ASTONISH. However, almost immediately after Kirby left COTU, two stories by one DC writer, France Herron, suddenly played around with the notion that two of the Challengers, Rocky and Red, quarreled a lot. The two stories-- "The Cave-Man Beast" and "Creatures of the Forbidden World"-- aren't especially well characterized beyond this schtick, probably borrowed from a similar one in the DOC SAVAGE pulps. But it's interesting that as soon as Kirby left the title, someone started a "meme" which Kirby and Lee would realize more fully in FANTASTIC FOUR's constant quarrels between the Torch and the Thing.