Not good!
Going NORDO after landing clearance is issued is very sloppy piloting. Even if the radio quits, you have two other radios. In a busy environment like SFO, if I don't hear anything for more than 30 seconds, I do a radio check!
I am sure they did not ignore the instructions deliberately, but c'mon fellas, were you trying to outdo your buddy and land on top of another aircraft?