Bought my 2021 G80 3.5 Prestige in July last year and have just shy of 5k on it now. I work from home so it doesn't get much action and a trip to St. Louis from Nashville has been the bulk of the miles really. With as little as it is driven, I'm surprised I haven't seen some of the battery issues others have noted, though it will not auto stop/start the engine till I've driven for 45+ minutes straight. The only real issue I've had is the head/navigation unit freezing and restarting randomly. The first time was in Bellville, IL on I64 and it happened twice along the same stretch so I chalked it up to the extremely poor lane markings that I could barely see. After that, it was a while before it happened again, and again, poor lane markings so I didn't think much of it. The last time it happened 2 times on the same drive in an area with very good markings so I took it to the dealer within an hour of it happening. They checked for codes and found some errors indicating a bad head unit, that was in November. The replacement head unit arrived last month and the car is at the dealer right now to have it replaced. Genesis concierge came and picked up the car, they didn't have a loaner at the time, but I didn't need it since I work from home, they will have it back to me tomorrow. While it hasn't happened again since then and I also updated the nav system that some said fixed the issue, I've had some other weirdness with the head unit that will hopefully be fixed.
Overall, I've had a great experience with my 2015 Genesis and the new one. I've had one issue with the dealership and the service advisor didn't last 3 days total before they fired him. Other than that, I've had all good experiences there in the 5+ years I've been using them. Even when my 2015 was totaled and I was in a crunch, I was able to get the new one for $69k OTD when they were going for 5k-10k over MSRP at the same store.