I have often heard that I should not use the Unlicense because of issues regarding putting things into the public domain. However, I do not understand why this would be an issue for the Unlicense. The Unlicense attempts to put whatever is being unlicensed into the public domain, and if that works, awesome! However, the author of the Unlicense understands that putting something into the public domain is not so simple, it may even be impossible, and therefore the Unlicense contains a backup clause (the 2nd paragraph) which clearly states that everyone is free to do whatever they want with the Unlicensed software. The Unlicense even includes a disclaimer containing the usual "this software is provided as-is blah blah" legalese.

Is the Unlicense bad because it is short and doesn't define who the "unlicensor", the "unlicensee" and Santa Claus is? If yes, then what about the MIT/BSD-style licenses? They are generally considered to be valid, so why isn't the Unlicense? Is the opposition to public domain waivers with permissive license backup clauses, such as the Unlicense, and even the Creative Commons CC0, just FUD or are there really major legal issues with them?

Here is the full text of the Unlicense:


  This is free and unencumbered software released into the public
  domain.
  
  Anyone is free to copy, modify, publish, use, compile, sell, or
  distribute this software, either in source code form or as a compiled
  binary, for any purpose, commercial or non-commercial, and by any
  means.
  
  In jurisdictions that recognize copyright laws, the author or authors
  of this software dedicate any and all copyright interest in the
  software to the public domain. We make this dedication for the benefit
  of the public at large and to the detriment of our heirs and
  successors. We intend this dedication to be an overt act of
  relinquishment in perpetuity of all present and future rights to this
  software under copyright law.
  
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
  IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
  OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
  ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
  OTHER DEALINGS IN THE SOFTWARE.
  
  For more information, please refer to http://unlicense.org/


(Disclaimer: IANAL - for reliable advice on legal issues, ask a lawyer.)
See the discussion on the OSI mailing list for some of the immediate issues with the license. My interpretation:

It's not global. It doesn't make sense outside of a commonwealth ecosystem,  is explicitly illegal in some places (Germany), and of unclear legality in others (Australia)
It's inconsistent. Some of the warranty terms cannot, logically, co-exist, given the current legal ecosystem, as written, with the licensing terms.
Its applicability is unpredictable. The license is short, clearly expressing intent, at the cost of not carefully addressing common license, copy-right and warranty issues. It leaves a lot of leeway interpretation - meaning that, in the US, it will take a few trials before you can reliably know when the license is applicable, and how.

Personally, I think of the license as having been written in human-readable pseudo-code, without having been properly compiled yet to a given set of legal systems.
