Details are not a problem with high ISO settings, even when using jpegs. This image was made with the Fujifilm X-T2 and the 55-200mm lens at 200mm focal length. It was captured handheld as a jpeg at ISO 12800, f/4.8, and 1/40 sec. The square image is an enlarged crop from the original image. No other adjustments were made to the out-of-camera jpeg.
As an aside, I bought this X-T2 new after the X-T3 was announced. I saw no extra value in getting the X-T3 for the types of photography that I do.