* There's no knife dueling in self defense - Alot of knife fighting in Filipino martial arts is geared towards knife dueling. Realistically, someone who intends on attacking you isn't going to wave a knife around and challenge you. He or she is going to surprise you by having the knife concealed then draw it and attack when you least expect it. By the time you realize the danger, it's too late. There's a difference between knife duels where you know it's going to be a fight and a knife attack where you probably won't expect it.
* Unrealistic knife attacks - In some martial arts self defense demonstrations, you might see someone put everything in one stab as you are taught a technique. In real knife attacks, your knife attacker is probably going to repeatedly stab you in a "sewing machine" fashion at various speeds. He or she might grapple to hold you in a firm position as he or she stabs or slashes away. Compare the knife attacks you see in Filipino martial arts to the knife attacks that have actually happened in the street.
* No situational awareness or distance control being taught - Very often, you will find a knife attacker conceal a knife before deploying it at the moment of attack. Situational awareness and reading body language are what's so crucial in saving your life in a knife attack. Someone in a crowd or store can walk close to you before quickly drawing a knife and slitting your throat before you'll have time to react. You surely won't have time to draw your own knife if you don't know how to create distance. If you don't know how to read a potential attacker's body language in order to recognize the threat, you will get seriously hurt or killed. Situational awareness and reading body language are two essential skills that you won't learn in a martial arts setting.
* Ineffective sparring sessions - Some schools like to teach point systems and rules (ex. no striking the face) or only making light contact with your knife. The result is when a real violent situation, your slashes are not going to be powerful enough to cut through muscles and tendons. They are only going to cut your attacker. Point systems are useless if they don't teach you how to end the threat efficiently. There's no rules out in the streets so following rules in training will only limit you when an attack happens in the real world.
There you have it. Knife fighting is alot more chaotic and intense than martial artists make it out to be. You don't have to take my word for it. Test it out in unrehearsed sparring sessions as you mimic an untrained knife attacker repeatedly stabbing you quickly as you try to apply Filipino martial arts then see the results. Knife-fighting is not a dance. It's a matter of life and death.