I've done it twice so far. In both cases I tried by myself through the website but wasn't able to complete the transfer - got an error message telling me that my A3 account number wasn't valid (lol). I had to call Bonvoy and then the agents had to put me on hold - again in both cases - because they weren't able to complete the process as well! In the end they processed the transfer and I saw the miles in my Miles & Bonus account the next working day (I had been told that it could take up to a week or two).
The fact that I have to give them a call is a bit annoying (same goes for teh waiting time of 30-60 mins) but at least it works...