I have a 820 and a JVC NX.
My understanding from reading Kris’s review and the comments in the JVC owner thread is that for most titles both modes offer equivalent (great) performance, but that for some titles, the basic curve needs some manual tweaks, and the high luminance one disengages the BT2020 P3 color filter.
Kris made it clear that this was disc-dependent, not setting-dependent.
“Double tone-mapping” (which I use) is a fire-and-forget solution, where you can count on a great PQ for all titles.
My interpretation of what Kris wrote in this forum is that for some consistently dark titles (e.g. Blade Runner 2049) you could possibly get even better results by tweaking settings using the Basic Luminance curve in the JVC, or possibly using the 350 nits setting in the UB9000, but this would apply to this title only.
On the opposite side of brightness, my own personal guess is that when applying a 350 nits curve on a “too bright” title, you could risk clipping highlights too much unnecessarily.
The double tone mapping with the 500 nits curve is a “safe” middle ground.