I should pull my finger out and publish some of the long term results with the ltzs. Honestly I think that quite a bit of it comes down to luck - did you get a good ltz and just the right resistor combo for it. I have multiple combinations of pcbs, op amps, resistors and ltz, I think about 70% suck (10+ppm overall true accuracy over the long term - including the box, pcb, reasonable room temp etc) , the others pretty good ie within 2-4ppm long term - as far as I can tell. Looking at all of the references - the 3458 itself looks pretty good - it is an old unit - and I think HP got first dibs on the ltzs and cherry picked the better ones.
I have only had the 3458 calibrated once and it did not seem to like the travel! All its measurements of most of my ltzs were out by 1-2ppm for a month or so after it returned then settled back to pre-cal. It was within spec and no adjustments.
Unfortunately the unit that TiN calibrated for me was not one of the better units.