One good way to see what the real story is in practical terms is caps-aholic http://www.caps-a-holic.com/all.php
Compare UHD to blu-ray and see.
In a few rare cases, some listed as 2K DI, still show very considerably more real detail than the blu-ray, such as with The Shallows (almost makes you think they went back and used the 3.4K capture instead of the 2K DI for all of the many scenes without SFX, although maybe the blu-ray was just that bad, perhaps the latter is more likely since the former is so rarely done). Many of the 2K DI also show more detail, but just a trace and there are few that almost arguably less to have less detail than the blu-ray, if anything (Serenity). There are some that are mix and match, The Martian (any scene with no SFX is true 4k, ones with SFX are 2k DI, the latter is most scenes though) or say The Mummy (but this has a lot of scenes that end up true) or Ex Machina (this one is interesting since even many of the SFX stuff seems to show a lot more detail, not sure how that can be, maybe the blu-ray was total junk or they did redo more stuff in 4k?).
Then for the true 4k titles, some shows tons more detail than 1080P ever could, others only modestly more. Some of these ones look vastly more detailed than the blu-ray (one example is a few of the Mission Impossible titles which have crazily more detail than the blu-rays, although one of the true 4k MI only has a trace more real detail for whatever reason and the 2k DI MI ones don't really have any more detail at all). One odd one is that Annihilation is listed as 4K DI on those sites, but the one comparison I've seen shows no more detail whatsoever than the blu-ray, although the color saturation and everything is remarkably better and richer and more like the theatrical presentation. Quite a few of the true 4k ones do show a lot more detail though.
One curious one is Terminator 2 UHD which got so slogged I hadn't even bothered to give it a second thought. However, it actually shows vastly more detail than any U.S. blu-ray release has and even noticeably more than the recentish special remastered overseas Studio Canal blu-ray which itself shows a lot more detail than any U.S. release. Of course it still does have the get rid of all film grain look that most hate so that is why it got slagged. But the slagging made it sound utterly useless and yet it's clearly, grain lacking problem and all, still clearly the most detailed option for it ever released and at only $12 bucks now seems like a no brainer.