I run onyx every week on 4 or 5 toons now that they can solo it and still nothing. I ran Kara every week for years on my main mount hunting toon before the account wide stuff went into effect and nothing. Then I started to run it weekly on all my toons and got it the second week. Rivendare took months of farming as well until it dropped on my main mount toon (again before the account wide change), ravenlord I farmed with a druid friend back in the day so have had that for a while. Now working on Zul raptor every day on my tank class toons until it drops.
I have to say the account wide thing is great for mount hunters because it allows more chances and the use of a wider range of classes. However, those ultra rare mounts are now all over the place because anyone can use it on any toon they have on any realm the want. Sort of takes some of the fun out of it.