I don't claim to know in the scientific sense why the Viv tonearm sounds very good with every cartridge, but here is some food for though:
(1) TAE. While the Viv and all other underhung tonearms with zero headshell offset does exhibit much higher TAE than can be achieved with an overhung/offset headshell, there are some mitigating factors, even assuming TAE is a major determinant of high SQ. For example, my 9-inch Viv would be expected to exhibit about 9 degrees of TAE at the outermost grooves and about -9 degrees of TAE at the innermost grooves, assuming the playing surface of the average LP is about 3 inches across (the radius of the LP from outer to spindle). This is assuming you set up the tonearm such that the single null point occurs in the center or middle of the playable surface. At that point, TAE=0. Thus TAE is very gradually changing from +9 degrees down to zero degrees and then further "down" to -9 degrees near the runout grooves. The change in TAE is linear (but on the arc of the stylus). If you consider only the middle inch of the playable surface, TAE goes from about 3 degrees through the zero null point to -3 degrees. This is about what you get with a well aligned conventional tonearm. Possibly, the continuousness of the sound from the Viv has to do with the linear nature of the change in TAE. Conventional tonearms generate TAE that goes up and down and up and down across the surface of an LP. Maybe that is not so good, even though lower in magnitude than a UH tonearm.
(2) Skating. The skating force generated by the Viv and other UH tonearms is directly proportional to TAE, because the headshell does not add to the skating force. Whereas, for conventional tonearms, the headshell offset angle is the major cause of skating PLUS the effect of any TAE. It thus has been shown that a conventional 9-inch tonearm generates about 2.5 to 3X more skating force than does a 9-inch UH tonearm. And just as with TAE, the side force generated by a UH tonearm has its maxima at the outermost and innermost grooves, but at the null point, the direction of the side force changes by 180 degrees, pulling the tonearm outward instead of inwards. This makes the side force very low on either side (outer vs inner) of the null point. Yes, we correct for the skating force of conventional tonearms with the application of AS, and we all know how imperfect that is. Moreover, AS is applied back near the pivot whereas skating happens at the stylus. This puts a force on the fulcrum of the cantilever that may be a source of distortion in overhung tonearms.
These are my thoughts. Raul says I cannot justify what I hear from the Viv in "audiophile" terms. (I won't sully the word "scientific".) But there actually are things to think about here.