Smorton:
It depends on what you do (hobby vs. commercial, wild-life/sports vs. travel, etc.). Nikon FX (D810, D750) to any DX including X-T2 is a step down (just my view, others may disagree) in terms of focus accuracy, depth of field, and overall image quality (not to say Nikon raw files are much easier to work with in LR than Fuji's). The difference is quite noticeable and I have seen this first hand. Ideally, X-T2 can be an excellent addition to a D810 or D750 setup rather than substitution, of course, if money permits. Again, my personal view, not trying to upset anyone.
However, FX lenses (good quality, f1.4 or f1.8 or f2.8) are much bigger/heavier and so is the FX DSLR camera and flash. I shoot family and travel and don't make money out of photography. For me, X-T2 + lenses are easy to carry and less eye-catching (than Nikon setup). The pictures are also excellent, colors are great and the move (Nikon FX DSLR to Fuji DX mirrorless) serves my purpose. After all, moments, subjects and light play a much bigger role in how memorable your picture will be than the hardware one uses. What I don't like about X-T2 (coming from a decade worth of using FX) is native ISO of 200 (which I would have liked to be at least 100), and not being able to use as fast a shutter speed for a given aperture as on FX. Lighting system is also missing with Fuji though EF-X500 is a good first step.
Good luck.
EDIT: By the way, if you never had a good FX camera setup, you won't have any previous bias/way to compare. Make no mistake, X-T2 is a great camera and Fuji has excellent lens lineup. You'll love the pictures you get from X-T2.