The key difference between Trifield and L7, PLII, CS, Neo, etc, is that Trifield is passive--that is, no steering logic. It results in good stability as one might expect, but as frequency changes, so can the image location due to the different frequency shaping applied to L/R outputs compared to the C output. I have heard an instrument move laterally as it traverses the scale (not always, but just to say every process has it's side effects). It also imparts a small degree of timbre shift in the front channels and also from the low-pass filtered surrounds as compared with the 2-ch source. Because the surrounds are passively derived, this layer of coloration is subtle, but omnipresent.
Having said that, it was Meridian's Trifield and Music Surround modes that finally convinced me that when processed well, listening to music in 5.1 could be one's standard mode. Switching back to stereo became a disappointment. It was this benchmark that PLII had to address, and was used as a reference during its development.