Which professionals do you consider least trustworthy? Car salesmen? Politicians? Telemarketers? Bloggers, maybe? Let me suggest a new addition to your list: you simply cannot trust a knife expert with no scars.
This is the consensus view among self-defense instructors: if you are attacked with a knife, you will get cut. You should expect to get cut. Your goal is not so much to avoid getting cut, but to avoid getting killed.
So next time you meet a self-defense expert, look at their arms. Do you see any knife scars? Have they even once tested their theories against a real, razor-sharp blade?
British raconteur and martial artist Chris Crudelli managed to find one Escrima teacher with sufficient courage to test himself—on camera, no less.