Lothrik, a dataminer who previously made accurate build planners before Diablo 4 was out, has released a sheet of data that supposedly shows the likelihood you'll find an Über Unique in the wild. Blizzard hasn't confirmed whether the data is correct, but the numbers line up with D4 Craft's datamined info from last year.
Diablo 4 streamer Rob2628 summarized the findings in a video and the takeaway is that the most recent patch didn't increase the drop rates significantly enough for anyone to reliably find them—Über Uniques went from absolutely impossible to drop for the most dedicated players to very unlikely to drop for the most dedicated players.
Each class has different drop rates because they [[link]] can't equip every Über Unique, but barbarians, for example, went from a one in almost 10 million chance to see a Harlequin Crest (AKA "Shako") to a 1 in almost 7,000 chance. So if you want to spend hundreds of hours spamming dungeons, go ahead, but it makes the dozens of hours to farm Duriel (a boss who has a 2% chance to drop them) look generous.
Scattered reports from players who have found Über Uniques after the most recent patch line up with Lothrik's data. Über Uniques do in fact drop outside of Duriel now, but you're still better off killing him over and over again than anything else.
Players had hoped for a way to break up the monotony of farming materials to summon the same boss for the entire three-month season. Über Uniques aren't necessary for most of the game, but they have fun effects to build around, like the amulet that turns your mana pool into your health, granting you near-immortality. But it seems that Blizzard doesn't want just anyone running around with these powerful items.
It's a confusing stance to take when a new crafting method [[link]] was just added that lets you take five Über Uniques and turn them into one of your choosing. Very few people manage to find one, let alone five, so Diablo 4 players have spent the last few days wondering [[link]] who the system is for.
Season 4 will bring huge changes to how items work in an effort to save you time sorting through them in your inventory. It's possible Blizzard has bigger plans for Über Uniques to come out then, but for now, you'll just have to keep grinding and hope that you get lucky.