TK, I agree with you that people shouldn't just pull something out of thin air because they've read/heard/saw a Yourtube on it. If someone hasn't taken at least some weekend seminar on knife defense/offense, better yet several or even years with a school, you don't have much of a valid basis for an opinion. Those of us that have taken several such seminars or invested some time in more extended training usually have a few such folks to recommend.
I know the Janich, Shivworks and Mercop programs and the people and they're each as good as anything out there. Mercop is going to leave you with a lot more bruises than Southnarc or Michael, but George thinks that you won't be able to perform for real if the training isn't as real as the encounter is likely to be. Better to get bruised up than bleed out. You'll get your cardio in any of them!
So, while I've not recommended anything specific to date, I'll recommend that anyone interested in just taking a couple of weekend seminars take the Shivworks or Mercop or Janich courses where you can find them. If folks are interested in a long term course of study there are a lot of knife/MA traditions to choose from if you're willing to take 3 years of twice a week to study them with a good trainer.
As to Krav Maga, I've never studied it, but I have "played" with people that have invested some time with it.
BTW, videos may be ok to watch, but training in a contact activity like knives, or any martial art, really requires you to have contact with another person. To get any good you should have a trainer. Since this isn't knitting, If you're serious about this for self defense instead of just an exercise entertainment, I'd advise investing the money in a trainer that knows what they're doing. That might be a weekend with one of the aforementioned nationally recognized trainers or a year of twice a week at a local school.