Disney cartoons are all about character, while Ub's studio's cartoons were expertly crafted they just never had characters that the audience could identify with or really like.

Ub was a fantastic illustrator, as well as a very innovative technician, but Disney's sense of storytelling and character development helped elevate his product above the bulk of what others were doing during those early days of sound animation.  (And thanks, Spyclops, for bringing us that second skeleton short!)

If I recall correctly, wasn't the bomber segment of HEAVY METAL scripted by Dan O'Bannon?  Seems like it took its cue from an earlier idea O'Bannon had for a bomber tale about the crew trapped in the plane.  The original concept had, if I recall, gremlins.  O'Bannon had previously taken that unfinished idea, along with a second concept he had on file, and incorporated the two into his original screenplay for ALIEN.