That might be true for the D810 and D850, but not necessarily for other Nikon DSLRs (or other brands for that matter), where the lowest ISO ("regular" ISO, not talking about Lo-Settings) does not necessarily give the best results.
I agree. I tend to generalize.
But with the Nikon cameras I've used, the base ISO is almost always the cleanest, of corse I haven't used all Nikon cameras just the: d70,d200,d300,d7200,d700,d3,d800,d850.
Yes, you can do a lot with ND filters... but you need to buy them (probably several of them for different filter sizes) and carry them around.
And multiple exposures do not give you smoothened water or clouds, nor do they remove groups of tourists walking around your subject
The only time I use the low settings, is to beat back the sun for an hdr or to replace a ND filter for a long shutter. Seems to work ok for that. But I try to keep in mind that it might have less DR. Haven't tried on the D850 yet. I don't use it often. I don't know what would be better though, the low setting or the filter?