It's almost like the "there are Apple people and PC people" discussions. The answer is it all depends. Smaller boats and full keel boats can use windvanes much easier than larger boats and fin keel boats. You won't see windvanes on ocean racing boats anymore.

I dragged a Monitor around the world and almost never used it. Even if you have a windvane there will be times when you need an autopilot. For my boat the windvane just wasn't powerful enough for tradewind sailing - the following seas knock it off course and the windvane just can't keep up. Windvanes require that the boat be very well balanced which to me means depowered. Electric autopilots an handle lee or weather helm. They can also sail a truer course.

It is true that the downside it power consumption and reliability. That is why I have solar, wind and water generation capability. My last big passage was from New Zealand to Canada, three legs, 59 days at sea. Never ran the engine for electricity. And that includes refrigerator, freezer, Ham radio and a lot of other electronics. I also have fully redundant, hot switchable autopilots and a lot of spare parts. The servo is not replicated, it can be swapped out in less than 15 minutes with the spare.