There are a few four-way yields in my neighborhood, and I would guess that it is more often than not that they don't function as planned when two drivers approach the intersection at about the same time. Drivers generally seem aware of and willing to abide by a yield that occurs at an on- or off-ramp or some other place where two roads merge. At the four-way yield, however, I think that the typical driver's attitude is: "It's a yield, not a stop sign. Therefore, I don't have to stop." While technically correct, this ignores the whole point of the sign. One intersection in my neighborhood controlled by a four-way yield is the crossing of a somewhat busier street with a somewhat less busy street, and I have often seen drivers on the busier street cruise through the intersection totally unaware of drivers approaching the intersection from the cross street. That these intersections are mini-roundabouts with a big, round planting in the middle exacerbates the problem: even if the driver on the somewhat busier street notices the driver entering the intersection from the less busy street, the former driver tends to proceed into the intersection if he can do so before the other driver gets to his position, when the standard should actually be to yield the right of way to the driver that enters the intersection first. If the planting were not in the middle of the intersection, there would be no ambiguity about which car entered the intersection first and thus has the right of way.
Much better if these intersections had either two-way stops or four-way stops (or nothing). Whatever is this intermediate case that the four-way yield is supposed to cover, it isn't doing a good job of it.