I own an X-H1; recently bought.
I've used the X-T3 and coming from a DSLR I didn't get along with the ergonomics. It's okay-ish when you're shooting (not great of a grip) but when you're out and about, walking just holding the camera in one hand, it's not comfortable (at least for me).
Better autofocus, Eye-AF in continuous AF.
Newer sensor, and processor.
More resolution (2 megapixels if you care about such things).
compared to the X-H1 which has
Better Ergonomics, and will balance out heavy lenses.
Better rugged build.
IBIS (it's useful in photography too).
With the battery grip, it feels like one solid piece (like a pro body should).
X-T3 is a better camera overall. Just the autofocus alone can be a reason to buy that.
X-H1 is more like a personal thing. If you want those ergonomics. And if you care about IBIS.
Don't get me wrong. It is a really capable camera. Will probably outperform any regular (Not pro like the D5, 1DXii) DSLRs. But compared to the X-T3 it has a slightly inferior Autofocus system. And can't shoot insane 30fps Stills.