I'm running Ultegra di2 shifters with an XT rear derailleur on my Warbird. It shifts beautifully and has plenty of gearing with a 10-42 XX1 cassette and 42T front 1x chainring. I personally like the size and shape of the Ultegra hoods compared to the GRX. These days its all about what you can actually find to purchase in the current market and like others have said everything is pretty much mix and match except derailleurs. You must match the derailleurs by discipline so if using mtb derailleurs XT & XTR only but GRX will mix with Ultegra and Dura Ace. Shifters don't matter at all so use whatever you can find.