I don't think I've ever considered the availability to be good.
Don't get me wrong. I think the Avios scheme overall is good. But availability is only a part of that and I don't think it's good at all. I came over to BAEC from American, and I was used to their scheme which I would describe as good (At least then - may have changed since). There, you could pretty much find tickets in any month you wanted to travel, but may need to be flexible on specific days.