We've been using built-in type bearings since 2006... We bought-into the "Sloppy 7" story line which basically asserted that stacking tolerances was bad for speed. And that's still probably true, but Hamboards are about graceful turning not just speed. We have come to suspect that during repeated big heavy turns, like with a Hamboard, the built-in spacer type bearings are so precisely mated that they tend to apply intense side loads to the bearing components and alternatively push-out the inside bearings, eventually loosening both bearings from their snug fit in the wheel hubs. It makes sense because when any system is loaded, it deflects. It's super hard to see with the naked eye, but it happens. Repeated alternating deflection leads to wear and fatigue. This fatigue eventually results in bearing vibration (noise) and friction (drag). Built-in spacer bearings are awesome for lots of uses... especially for straight-away speed... but maybe not so great for big heavy turning like on a Hamboard or cruiser.
The SAN-O bearing 10.3 mm spacers give extra tolerance gap to help manage the tremendous side load deflections without wearing the bearings guts as much and without pushing-out the inside bearings as much... We think that the result is that SAN-O bearings handle turning better and they stay locked in the plastic wheel hubs better than built-in type bearings.