The caveat with the Unlicense is that it doesn't work in some jurisdictions, and the work may be considered literally unlicensed, as in nobody except the copyright owner can use it. In practical terms, of course, I doubt anyone using the Unlicense plans to come after you for copyright infringement, but it's something to keep in mind. This is why many organizations recommend instead using something like CC0, MIT etc.
The caveat with the Unlicense is that it doesn't work in some jurisdictions, and the work may be considered literally unlicensed, as in nobody except the copyright owner can use it. In practical terms, of course, I doubt anyone using the Unlicense plans to come after you for copyright infringement, but it's something to keep in mind. This is why many organizations recommend instead using something like CC0, MIT etc.