I am currently migrating a client’s data from a MacBook Pro to a new MacBook Air.
We have tried it twice over Wi-Fi and it failed both times, so I asked her to bring it over, and I did it using a hard drive as a middleman. This time is working flawlessly.
Historically, migrating from one Mac to another has been the easiest thing going. But since they started eliminating FireWire, and thus Target Disk Mode, from many Mac models, they have had to kludge a solution together. They went with Wi-Fi, rather than telling people to use a hard drive in between.
My own previous experiences with migrating over Wi-Fi have been less than stellar. But my contractor said it always worked for him, so I’ve run with telling people that.
In short, if you run into problems migrating over Wi-Fi, try doing it from your Time Machine backup. And if that doesn’t work we can do it with an external hard drive.