I recall hearing that SQLite actually had some significant issues with choosing public domain as their license and somewhat regret the decision. Apparently it’s not a concept which has broad understating internationally, and there’s less legal precedent in a software context which has made it harder for some teams to adopt due to concerns from legal departments.

The Unlicense isn't "just" public domain though, it also has a fallback clause that explicitly lists things you are allowed to do ("copy, modify, publish, use, compile, sell, or distribute"). So I think the intent is, even if PD isn't recognized and line 1 is invalid, you're still granting a license to the same effect.

SQLite on the other hand just says

    The author disclaims copyright to this source code.  In place of a legal
    notice, here is a blessing:

      May you do good and not evil.
      May you find forgiveness for yourself and forgive others.
      May you share freely, never taking more than you give.