The Unlicense is intended as a public domain dedication. I don’t think you should use it, but should prefer CC0 instead. Here’s why.
1. It’s poorly drafted in potentially problematic ways.
The vibe I get is that around 2011 people said “eww, your intent may be good but your wording is bad and it won’t hold water in some places, especially Europe”—it being drafted by a layman unfamiliar with European law; but that by 2018 or so they mellowed to “OK, so it’s not great, but it’ll probably work despite that—but CC0 is still better, prefer that instead”.
2. Its name is abysmal.
For me personally it’s actually not its drafting that leads me to despise the Unlicense, but its name, which is an atrociously bad choice. It’s fine when spelled with a definite article (“the Unlicense”), but once you remove the article and express it in other forms (as its website does multiple times and as people habitually do), it’s terrible:
The English term “unlicensed” means “all rights reserved”; that is, you have not been given license to use this. And would they have “Unlicensed” mean “no rights reserved”? [There’s quite a history of auto‐antonyms in English. My favourite is let, which has for a thousand‐odd years meant “permit”, but before that and until comparatively recently also meant the opposite, “hinder”—a sense that persists in only about two places: the legal expression “let or hindrance”, and the tennis concept of “let”, referring to the net obstructing (though not stopping) the ball. I don’t want “unlicensed” to become an auto-antonym.]
As noted, basically all relevant parties that have expressed an opinion recommend Creative Commons Zero (SPDX identifier CC0-1.0) instead, as better‐drafted and more thorough.
The OSI specifically have not expressed an opinion; I bet they’re all thinking it, but it’s a delicate matter that they can’t say out loud because they haven’t approved CC0. That looks to be more political than anything else; ⅌ Wikipedia’s summary: Creative Commons applied, some people got antsy about consequences of it explicitly saying patent rights are out of scope, and Creative Commons withdrew their submission—my guess without having read the precise history is that they got fed up with the silliness and didn’t care all that much about OSI approval anyway. And then some years later, OSI did eventually approve the Unlicense, which, making no mention of patents, is in precisely the same boat. Go figure. [One amusing result of all this is that the Switchmode Developer Agreement (part of a project led by the inestimable Kyle Mitchell) defines Open License as “a form public license approved by the Open Source Initiative as an open source license, or the Creative Commons CC0 1.0 Universal form license”.]
The matter of patents is particularly interesting because where patents may be involved, a public domain dedication like Unlicense or CC0-1.0 is actually less permissive than an attribution-only license that includes a patent grant, such as the Blue Oak Model License 1.0.0 [Again work led by Kyle Mitchell; seriously, check out everything he’s done, there’s a lot of interesting stuff there in the realm of software and making money and law.] (SPDX identifier BlueOak-1.0.0) which, incidentally, you should use instead of the likes of Apache-2.0 and MIT.