I’ve owned an iPhone as my primary mobile phone starting with the iPhone 3GS. In all that time, I believe the only time that iTunes WiFi sync ever worked reliably was the first time it was introduced in 2010. Since then, it’s been a nightmare.
Sometimes the phone and iTunes machines can’t discover each other over WiFi. This is presumably due to the utter clusterf*** that is Bonjour/ZeroConf discovery.
Let’s say you do get lucky enough to start syncing. The sync itself can literally just hang at any point. Sometimes you have to reboot the host machine and sometimes you have to reboot the phone, and then try again. Sometimes you have to do both.
Oh and syncing via the wire is a little more reliable, but not much. I still experience my fair share of hangs there too, but at least when I plug the phone in, it always tries to back up.
In summary, I’ve waited years for this feature to work and I assume by now it will never work properly. My dream is simply to look at my iTunes server and see that the most recent backup was today instead of a month ago … oops, guess it hasn’t been working … AGAIN! Because if anyone at Apple actually used this feature, they would figure out in a jiffy that it is completely unreliable and will, without fail, refuse to work if you don’t keep rebooting your devices and software constantly.