only a decade ago, an autopilot didn't do that good a job steering downwind in a seaway. A vane could do just as well, properly tuned. However a modern autopilot with gyros steers a much better course downwind than a windvane (or old autopilot) could ever hope to. A vane must wait for an error in course to develop before applying a steering correction, just as the old pilots did. However a modern pilot will apply a correction in response to yaw and roll accelerations, just as a good helmsman does, anticipating the course change and preventing any yaw momentum from building up. The difference between my old Raymarine 5000 and the 3GS was remarkable, and the X30 I have now is better still.

These days, I don't think the failure rate of a vane or pilot differs much - but the ability to repair a vane with baling wire and chewing gum makes it operationally more reliable.