I have a question about wifi-assist: If it kicks in doesn't it tell the phone wifi is off? I ask because iOS prevents lots of things from happening over a cellular connection. For example: You cannot do a system update, you cannot download apps larger than a certain file size (last I checked it was 50 megabytes), and apps like Netflix have a "don't use cellular" switch. It seems to me that, assuming that those switches aren't bypassed, the likelihood of a ridiculous bill seems minimal.
Pardon my skepticism, I'm one of the grandfathered unlimited customers with shitty wifi at work who is continually annoyed by these artificial limitations. I'd actually benefit from it if Apple went that far out of their way and fucked up that bad.