It might be cheaper/easier to build a unipivot, particularly because precision machining of the point and cup is not as critical as the machining of conventional gimbal bearings, but, some very good builders with very exacting machining make unipivot arms, and some very cheap arms are made with coventional bearings. I don't think cost explains the choice made. There are arms that do seem to be quite sloppy and chatter that are made with all sorts of designs.
As to the Naim ARO, that is an interesting arm. It may be a bit sloppy, and to my ears anyway "jangly" sounding, but I can see why some people like it for its lively character; I heard it perk up a system that sounded a bit too dark and dead sounding for my taste.
