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?
For the lazy people, 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/