Warning: SPOILERS for A.X.E: X-Men #1The mutant Cyclops has always feared Jean Grey - especially after the two X-Men were married. As members of Professor Xavier's first class of mutant students, Jean and Cyclops have battle experience to spare - and fought side-by-side for decades. They know each other incredibly well, but A.X.E: X-Men #1 reveals that Scott has always feared Jean's power...and he has every right to do so.
Cyclops and Jean Grey's relationship was fraught with tension from the start. All of the men in Xavier's class (including Xavier himself) pined for Jean, but Cyclops only found real competition in the form of Wolverine. The love triangle between the two mutants was only complicated further by the events of the Dark Phoenix Saga, in which Jean obtained new powers, was brainwashed into using said powers for evil, and broke free of the mind control only to hunger for more power, destroy a star and a civilization, and die for her great crimes. She eventually returned from the dead, but her time as the Dark Phoenix would haunt her for the rest of her days.
In A.X.E: X-Men #1 by Keiron Gillen with art by Francesco Mobili, Jean Grey leads a group of Avengers, X-Men and Eternals inside the Progenitor Celestial to destroy it from within. The group knows the Celestial administers surprise tests, and Jean's is no different: she's confronted with images from her past, including her time at Xavier's School for the Gifted, her tenure on the Quiet Council and her deeds as Dark Phoenix. The Celestial eventually talks to Jean directly; mentioning Cyclops advised it to be weary of Jean, it says "[Cyclops] is married to something that, on a fundamental level, petrifies him...he sleeps in the lair of a red dragon...and I think he's right. I think you are like me."
Jean Grey is Dangerous, And Cyclops Knows It
The Celestial is referring to Jean Grey's wave of destruction as the Dark Phoenix, and though she gave up the Phoenix Force long ago, she remains a truly powerful ally...or enemy. Jean was mostly innocent during the Dark Phoenix Saga - she was manipulated, used and corrupted practically from the first page - but many people in the Marvel Universe still pin one-hundred percent of the blame on her. Jean controls her powers more than most, but many remember her lack of control during Dark Phoenix...and so does Cyclops.
Cyclops is a comparatively weak mutant compared to Jean (in terms of power) and his wife could easily kill him and many others if she truly wanted to. Fortunately for all, she's a hero rather than a villain. Jean Grey's time as the Dark Phoenix was brief, but she is remembered for her evil acts rather than her good, which makes Cyclops weary of his wife, one of the most powerful X-Men of all time.