I have heard medium mass arms mistrack many records and then here the
same records on a mission mechanic arm and they track perfectly.
@tzh21y The issue here is something called 'effective mass' which is the combination of the mass of the arm and cartridge, and how that interacts with the mechanical resonance, which occurs when the effective mass interacts with the compliance of the cartridge.
Regardless of the quality of the arm and cartridge, if the mechanical resonance does not fall within the window of about 7-12Hz, the arm and cartridge combination will mistrack and you may experience breakup even with moderate modulation in the LP grooves.
IOW, 'medium mass arms' has nothing to do with it. What your post suggests is that the medium mass arms you heard were not set up properly.
The Triplanar allows some adjustment of the effective mass, and so it can accommodate a fairly wide range of cartridge compliance.