Holding a Nikon pro camera with a big lens like the 70-200 f.28 is definitely a two-handed job. However, it can be comfortable to rest your arm for a moment by letting your arm hang while holding the camera in one hand; the body and grip are big and so you can hold the camera/lens in one hand this way. With the X-T20 and 50-140mm f2.8, you can't really do that since the body has no real grip. Also, when two-handing the combination up to your face, the more weight you transfer to your camera hand the more the bottom edge of the camera body digs into your palm, unlike a big Nikon.
However, the 50-140mm Fuji has a very large zoom ring with lots of space between the ring and the body of the camera for your hand to rest. So the weight of the camera (just like on a big DSLR) and lens is primarily resting on the palm of the hand holding the lens. The Fuji lens is very ergonomic for this. The only problem I occasionally have is that I inadvertently nudge the aperture on the 50-150mm aperture ring (Nikon does not have an aperture ring on their 70-200).
I use the 50-140mm a lot on my X-T20 and I have no plans to get the grip or a wrist strap. The package is light enough to hold by the lens when one-handing it, and it won't cause a neck injury if slung to the side over the neck. I do use a monopod with this setup when I want to get video, and it works extremely well.
By the way, the quality of the 50-140mm on the X-T20 is outstanding; and the weight savings really adds up after carrying it around for an hour versus the Nikon DSLR and 70-200 equivalent.