I've never been to Japan before and I only have 10 days to do the Tokyo/Kyoto/Osaka loop. As much as I'd like to hit all 3 parks I can really only dedicate one day to theme parks in general.
I guess if USJ HHN has a night component or night ticket I might be able to do that as an evening activity. Not sure how HHN works there past it being a day experience as well as night time.
We ended up doing about 17/18 days in Japan the first time, some of those being travel days where we didn't really do too much. Of those days, we did four different park days (1 Disneyland, 1 DisneySea, and 2 Universal, one normal and one HHN).
We ended up starting with Disneyland our first full, non-jetlagged day, and it was a good starter to dip our toes into the waters for the trip with given it is very similar to the stateside parks. That being said, it's also comprised of a lot of offerings also seen stateside or elsewhere. Very very well maintained, and in almost all cases unchanged from opening day (Snow White has the witch at full blast, Pirates and Splash Mountain in their OG formats, etc), but also stuff you've likely seen before, with a few unique gems sprinkled in (Pooh, Monsters Inc, BatB).
Universal is in a similar space, but if you are there for HHN, you have the added benefit of seeing the park do both things simultaneously without a separate ticket. We did one day at Uni just plain, and another for HHN (we were in Japan over the course of HHN starting), and truthfully, the year we went (2024) we would have been fine packing it all into one day. Their HHN mostly opens with the parks (11am-12pm for some offerings), and so you have all day to experience the parks and the special offerings. Couple that with the fact that a lot of things require you to use the app and get into a lotto system, it means you have your special experiences pretty much pre-planned for you that day once you get there, and so you kind of re-construct your day based off of when you need to be available. The tough love part of this is, while I loved being able to check it off my bucket list, do not expect to compare it to HHN in the states. It is simply another beast. Far more vibes based, built around shows, and for us, there was only one true haunt. The rest were modified stunt shows and ride overlays. Again, super fun, but unless you absolutely value completionism over anything else, probably also better spent elsewhere.
DisneySea on the other hand, was a whole other ballgame. Best comparison is it is to Disneyland as IoA is to Studios in Florida, but even that comparison is reductive. We had a bit of a modified experience here too, in that Fantasy Springs was just opened, and we ended up not being able to snag entry times for anything beyond Peter Pan's and two of the restaurants, but it was still magical. Rides you may have seen before, presented in totally new lights, completely unique attractions, beautiful theming; just a wonderful park. If there is something you are looking for that is pretty much wholly unique, and you only have one day, this would be it for sure.