The ClearAudio designs actually seem to keep both horizontal & vertical effective moving masses low (and relatively similar) by attaching that small "stub" arm w/ headshell to a rails system. I think the downside here is need to keep the length of those rails clean over time, versus the non-issue this poses for a more traditional sealed/contained bearing. But air bearing linear trackers also have the issue of keeping clean.
I still have an ET2 linear air-bearing arm on an old Merrill Heirloom (in a closet unused now); that arm by its design definitely had a much higher horizontal moving mass. It was also a terror to use on a springy table like the Merrill without the optional silicon damping trough to calm down that frictionless air bearing with its high moving mass. However, when I could work up the nerve, it worked quite nicely with the old Ortofon Kontrapunkt series (medium-low compliance).
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