So is the donation with regards to the developer or a particular project? The developer doesn't get the donation with any regards to whether it is aimed at them or the project. Is the developer free to work on a different project? Does the person who donates even know? It just seems like it can lead to disappointment and frustration.
I believe I've seen some developers outright reject no-strings attached donations for this reason. What is expected of them? Should they feel beholden to those who give, and even if they shouldn't does psychology inevitably lead to that?
That's why I feel like it has to be hard goals. This is expected for this, and not a donation which has too many unintended consequences.
I don't mean to be overly argumentative.