So the IQ of Nikon lenses ends up being this enigma with no definition outside the usual comments.
There are lots of tests of many lenses - photozone, SLRgear, DPReview, etc. all test both Canon and Nikon lenses. There's no mystery-wrapped enigma, in general (although there are obviously exceptions) where both have a similar lens, the IQ of the Canon lens is better.
Now, if you specifically mean the supertele lenses, there aren't many tests of those from either brand. What tests there are favor Canon, which given their advantage at shorter focal lengths, use of fluorite elements, etc., makes logical sense.
Who knows - maybe the Nikon camp is running the tests, but are too embarrassed to publish the data... (kidding)
why use fluorite ? when Nikon have their "new SuperED glass" and the optical properties of this new glass closely resemble those of fluorite, Super ED glass is more resilient to rapid temperature changes (thermal shock) and not as susceptible to cracking as the crystal structure of fluorite. Super ED glass also boasts a higher refractive index than fluorite, making it highly capable of correcting aberrations other than chromatic aberration.
And fluorite is also one reason that NASA will not have anything to do with Canons " fluorite lenses" in the space.
Fluorite still offers superior dispersion control over ED/SuperED/UD/SuperUD elements. Canon has both UD and SuperUD glass, which is exactly the same as ED and SuperED, and they have clearly demonstrated the superiority of Fluorite over low dispersion glass for controlling the dispersion of light. SuperED glass is NOT better than Fluorite at correcting CA! It should also be noted that low dispersion glass only offers improved dispersion control...it is not inherently capable of significantly correcting other aberrations unless it is paired with other elements, or aspheric....however those solutions to other aberrations are not limited to low dispersion glass...any glass, or Fluorite, could be used for those purposes as well.
Fluorite IS more fragile, however not as fragile as many might think. Canon has buffered element mounts that reduce shock on the elements these days anyway, so cracking due to a drop or bump isn't a very big concern. Regarding thermal shock...that requires SHOCK, and that can only occur if the temperature changes VERY rapidly. Moving a lens into a house from outside on a cold night is not going to be cracking any fluorite lens elements any time soon.
Finally, you need to back up that last statement. Why wouldn't NASA take a Canon lens with fluorite into space? To my knowledge, none of the Nikon equipment has ever been used outside the spacecraft...so there is no need to be concerned about thermal shock. Nikon's newest lenses are beginning to employ fluorite (anything with the FL moniker is a lens that uses Fluorite elements)...I'd guess because otherwise they could not compete with the kind of IQ Canon is currently pumping out with their fluorite lenses. An example of a fluorite lens from Nikon is the new 800mm, which uses two fluorite elements. So, according to your anecdote...NASA won't be able to use future Nikon lenses either...right?