First: kudos to branadic, Kleinstein and some other gurus here (I mean that sincerely) for their work, knowledge and experience!
I'm a modest and pragmatic guy and am not even considering going beyond 1 ppm (with meters) and hence I'm not really interested in anything beyond honest and true 6.5 digits, which just so happen to be 1 ppm - but still I'm reading with attention and interest almost any and everything certain users (like the above mentioned) do and write about ... and learn from.
Here is my personal take on this topic:
I studied the ADR. Carefully. But I stick with the LTZ. Yes, in a few parameters the ADR is more attractive, noise being one of them. But for me, and presumably most hobbyists and people outside of cal labs, the "problems" the ADR solves are, excuse me, in between luxury and irrelevant. Why?
What I (I mean 'we' but I do not want to arrogate to speak for others) really need is long-term stability with acceptable to good precision and accuracy. Simple as that. And in that the LTZ still is unsurpassed. In addition it is well known and understood and "battle-proven" over decades.
Being in that department let me address another issue: I quite like the diverse long-term studies some of you made. But I don't care. At all. For two main reasons, a) Fluke, Datron, etc. make/made a living building the very best possible/feasible, and I guess trying to do better than them is quite futile (for mere mortals without a very serious budget), and b) how to measure (as opposed to guesstimate) and verify it? With an '8.5 digits' multimeter? Haha, good joke.
Btw, just look at that "new" (well, not really old) brit company "time" (or similar) who seems to be somewhat of a wannabe Datron "new edition" - and who seemed to fail and still be failing to achieve what Datron achieved decades ago.
Plus, again, IMO our problem isn't a few uV more or less noise. Our problem is 1 ppm per year stability.
Where I see the ADR is in places, for which the LTZ was overkill but LM399 isn't quite up to par that is, 7.5 and better 6.5 digits meters and not-quite-top reference boxes.
Again, thanks for your work. At the very least it helps us to better reflect those matters.