Kickboxing vs Boxing
Boxers who cross over are usually better with their hands than anyone in the room and lose anyway, because every assumption their footwork rests on is now false.
Boxing's hand technique is the deepest in combat sports. A trained boxer entering a kickboxing gym will out-jab, out-slip and out-counter almost everyone there, and none of that is in dispute. The problem is everything the hands are attached to.
Boxing footwork assumes nothing threatens the legs. A boxer steps freely with either foot, plants weight on the lead leg to load a hook, and slips laterally with the head moving into a space that in kickboxing contains a shin. Each of those habits is efficient in boxing and expensive in kickboxing. The lead leg becomes a target, the slip runs into a head kick, and the low stance that makes a boxer hard to hit makes them easy to kick.
Range is the second problem. A round kick has more reach than any punch, so the distance at which a boxer feels safe — just outside punching range — is the exact distance at which a kickboxer is most dangerous. Boxers crossing over consistently get caught in the gap between the two ranges, because in boxing that gap does not exist.
What transfers, and it transfers well, is the hand game itself: the jab, the counter, head movement against punches, and an understanding of ring geometry. The Dutch style is partly built on borrowed boxing mechanics for exactly this reason.
Running the comparison the other way is less flattering to kickboxing. A kickboxer entering a boxing ring loses the kicks that make their range work and finds their hand technique is merely good rather than exceptional. Neither art is a superset of the other — they are optimised for different rulebooks, and each looks incomplete inside the other's.