Watching the video several times I did not once see uke head hit the ground. Sure it is a hard style but uke's ability was up to the task. Notice that uke got up every time and did not appear to be hurt.
That's because it is happening so fast and so violently. Look again at the beginning. You can see uke's head bouncing upward violently after his body has already completely landed. One's head would not come up like that if it wasn't bouncing off the ground.
Getting your head bounced off the mat isn't necessarily going to disable you immediately or keep you from getting up, but it can accumulate. Someone posted a record of all the Aikido training deaths up until some time in the 80s and most of them were from head trauma caused by accelerating uke down into the mat repeatedly.
Personally, I don't see the purpose of accelerating someone down once they are falling unless you are trying to hurt them. You already have control of the situation and they are already falling down. Also, I find that being slammed down kinda pisses me off on a gut level, making it seem more likely to escalate a situation than diffuse it.
If you want to do something to someone on the way down, look into Systema. They have a way of hitting you up to several times on the way down that doesn't necessarily cause harm or rile you up.