This is part of why I have a lot of ambivalence regarding teaching unarmed defenses against weapons.
I have a reasonably solid repertoire of gun/knife/stick defenses. I've even had some success sparring unarmed against training weapons. However:
1) Any such techniques are inherently far, far, far less tested than the rest of what I teach. For example, if I'm teaching a right cross: I've landed multiple right crosses in real fights. My teachers have done the same. Through personal experience, first hand observation or video, we can examine potentially hundreds of thousands of right crosses in different contexts which have succeeded or failed and draw our conclusions regarding what works best. Contrariwise, very few instructors have ever performed an unarmed disarm of a real knife in a real assault. Of those who have, none have had occasion to do so more than a few times at most. Most of those rare occasions have not been witnessed or videoed. We don't know how typical those successes are or what sort of factors may have made a difference. We have even less information regarding the knife disarm attempts that failed. In short, we don't have the dataset for anyone to honestly claim to be an expert on the subject. (That said, I do know instructors who I think are on the right track and other instructors who I think are teaching garbage that will get you killed.)
2) The person using the weapon has a huge advantage. If the student learns unarmed techniques against the weapon without a) truly understanding how big their disadvantage is and b) understanding when the correct time and place to resist is, then they may be reducing their survivability by increasing the odds that they will try to fight when they shouldn't.
3) The success I've had in sparring unarmed against weapons doesn't come down primarily to the specific techniques I know. It has a lot more to do with my overall skills regarding distancing, body control, leverage, balance, timing, adaptability, etc, that I've learned in years of live sparring (armed and unarmed). A beginner who tries to apply a knife disarm without having that foundation is going to lose pretty reliably.
On the other hand, situations do exist where someone may need to defend themselves unarmed against a weapon and having at least some appropriate tactics on hand might help. I haven't worked out yet the ideal time to cover such material, but right now I'm leaning towards addressing it with students who already have a solid foundation of usable unarmed and/or armed fighting skills.