AFAIC, 90% of the knife-disarm techniques taught in the MA world are optimistic at best and suicidal at worst. If practicioners of these arts were to try their disarms against an opponent who was attacking with realistic attack energy and actually trying to stab/slash them (for example, the clip MJS posted from the Rich Dimitri seminar) not just feeding them the prescribed attack their technique would go down the crapper.
Yes, getting hit by the opponent's other hand is a concern, however the primary concern is NOT getting hit by the knife. As others have pointed out, in the clips from the original post, the defender was in the worst place he could be. He should have either broken contact, or "crashed" the attacker after getting the knife. From the attacker's perspective, it's gonna be hard to fight when you're getting pushed backwards and being blasted with knees and headbutts.
As someone else said, there is always a counter. There is not a technique in existence that is foolproof (unless you count a 12-gauge from 20 feet). If there were, that'd be all we'd need to learn, at least until someone came up with a way to defeat our perfect techniqe. There is always a risk or, dare I say it, a high probability of getting stabbed/slashed when you are forced to go empty-hand vs. knife. Anyone that says otherwise is an idiot. You may get lucky and take the other guy out without being injured yourself but I wouldn't count on it.
I personally feel that getting both hands on the knife arm in an effort to control the blade is the best way to avoid getting stabbed. Once that's accomplished, don't just stand there, ATTACK the other guy!!! I tell my students that their mindset in this type of situation should be such that they are willing to rip the other guys throat out with their teeth if necessary. Whatever it takes, take the other guy out.
If you want some good empty hand vs. knife material, check out Richard Dimitri, Karl Transwell's S.T.A.B. system (these first two were the ones in the clips MJS posted), Jerry Wetzel's RED ZONE, and Marc Denny's "Dog Catcher" (Marc is one of the founding members of the Dog Brothers).
Just my $0.02