Hi,
If by "mythical unicorn" you mean "something that does not currently ecist, and may never" I'd guess maybe something like a "fully accurate" - insert name here, would be my thought, in that I suspect that no matter what any company puts out, there will likely be those who don't agree with certain details.
For big stuff and/or one off items, like a specific ship (or something similar) I'd suspect that its actual appearance on any given day may have varied, so that just because someone may have a picture showing it configured one way on one specific day does not mean it might not have looked slightly different on another day.
As for other items where there were lots of a given item produced (like planes, cars, tanks, etc) I suspect that there may be small variations in each item produced that may result in variations in detail, eventhough all the items were notionally the same "variant" or "model number", etc.
As such, my guess is that no matter how hard a model company may try and research an item, there will always be debate on any given kits accuracy.
PF
PS. As an example, my last car apparently was a very early build Fiat 500 Pop, and I later found out that within just a few weeks or so after mine was built, the compnay decided to make some changes in the standard options, resulting in other notionally similar models on the lot having some differences in details (like the steering wheel, etc). As such, I always had the feeling that if I ever tried to build a model of that car, someone would look at it and tell me that I had gotten some detail wrong, since it didn't match pictures on the Fiat website