This is difficult because it has been left as purely a judgment call. In the clip by WCFB, the only potential foul is UNR. (You could maybe make a case for spearing, but I'm pretty sure the defender's helmet is to the side of the receiver. For the sake of argument, let's say the hit is NOT with the helmet.)
So, this leaves you with: Is it unnecessary or excessive? Does the defender have any other way to make a play? Is there any "extra" action that is mostly about making a hit instead of making a play?
For me, from the angle shown, I would say this one is close - I'm not actually sure what I would do with it in a game situation. Heck, I'm not even 100% sure what I *want* me to do with it in a game situation... but ultimately, I think this should be a flag for the way the receiver is slammed into the ground. (I can hear the coach now: "He's making a tackle - isn't that what we're supposed to be doing?!") The initial hit appears to me to be a hard but legal attempt to prevent the reception; others will certainly disagree with me. (Opposite coach, screaming: "THAT'S A DEFENSELESS PLAYER!")
Ultimately, though, it's a judgment call.