[TomasBM]

Actually, you provided an example where the obstacle was somehow surmounted [1].

The expectation doesn't have to be too specific or unrealistic. If you agree on some common ground [2], everything else can be fair game for the artist.

Your analogy with the bridge would apply if art also had a minimum viable version. Collapsed to its functional requirements, you could say that visual art is something to look at. But I doubt either party, especially the funding body or the public, would be happy without inserting some quality requirements (i.e., what makes something nice to look at).

Many artists do commissions, so you can see this as a commission with deliberately underspecified requirements.

[1] I won't get into the disagreements between the Pope and Michelangelo, and it's certainly not an example of a good contract, but we can assume that both parties were somewhat satisfied in the end.

[2] For example, both parties need to like it. Or the patron doesn't have to like it, but it needs to appeal to some public audience.