The problem is the automatic trim control system that was hacked on to make up for the design being fundamentally flawed (having engines too large for the obsolete airframe).

Edit: Why is this being down-voted? Is it factually incorrect or does it make people uncomfortable?

Yes, they have two less major mechanical systems. But they likely have more electronic and software systems, which tend to have more failure modes (and be more difficult to test) than their mechanical counterparts.

Back when I worked in automotive assembly, we'd did some work replacing mechanical systems with electronic controls. To a large extent, the mechanical systems, once tuned, would run indefinitely until they developed some mechanical wear. The electronic counterparts had to be re-tuned on a regular basis. The electronic ones had less moving parts, but increased complexity.