Granted I’ve not read Eric’s book, but from my understanding of anatomy.
The shoulders are inherently unstable joints, in which stability comes from the muscles and soft tissues (ligaments) rather than the bony structure. This is different from the hip joint, which is a “mobile” joint (refer to Boyle’s joint-by-joint approach) but is much more stable due to the higher socket:ball ratio. So increased muscular activity increases stability at the shoulder joint at a higher rate than the hip joint.
When doing unstable work with the upper body the limiting factor of stability is usually the shoulders than the wrists (elbows and knees are stable joints). With lower body work, it’s actually the ankles that’s the limiting factor. Doing stability work for the upper body enhances shoulder performance which is what we want. Stability work for the lower body might overcome our ankle stability threshold to give a “stimulus” to the core/hip stability, which might lead to an ankle injury.
I’m not too sure of the first point, considering that I’m still new to lifting. I would think that the former is “safer”/easier to control. That’s probably as the latter uses more of the core to stabilize yourself first before you can move the load. With the former, the “unstability” is targeted more to the joint you want to work specifically, as you don’t have to stabilize yourself first.