Conventional wisdom is that CK doesn't have much of an arm. And in fact we've definitely seen a bunch of under thrown balls from him down the field. And yet I was just
watching highlights from his Texans days and see a number of 50-yds-in-the-air throws. At the 5:17 mark he uncorks one that goes 60 in the air. And, even there, while he def winds up, a defender in running in on him so he doesn't get to step into it.
He certainly doesn't have an Aaron Rodgers or even JGoff arm. But watching the highlights I'm thinking that his under throws aren't simply or even primarily about lack of arm strength.