I have GRX (812) on the gravel bike but I haven’t ever used hydro Ultegra – although I have used rim brake Ultegra for years.
I usually find braking hard with brifters on the hoods makes my finger joints feel like I’m contorting my hand in a way that’s both slightly uncomfortable and mechanically disadvantageous. It’s not painful or anything, it’s just a bit unnatural. Which I think is why the GRX levers are said to be optimised for use on the hoods, and in that respect they certainly feel… fine. Better than the rim brake brifters for sure.
The other thing I definitely notice is that the GRX hoods are deeply, err, ribbed. This is loads better for grip in the wet mud but a bit annoying for using on long (road) rides without gloves in the dry. It’s a small difference but I’d definitely prefer some smoother grips overall.
Ultimately I reckon it (unsurprisingly) comes down to what you use the bike for. For long trips of mostly road riding, Ultegra. For a sloppy techy off road mud fest, GRX. For anything in between, realistically either will be fine.