Well, the biggest obvious flaw is that it's a knife. You might mangle the hell out of an attacker, and he might die; but he'll probably die later.
Angry people who attack normal folks for no good reason are frequently very hard to stop, and might not even notice that the pain they are experiencing during their attack is their flesh being laid open. Or they don't care.
A knife is better than nothing for damned sure, but it's not really a good defensive weapon, and you can't even stab with a Civilian. Which may well be your only option if you are rolling around on the ground fighting with a badguy. Don't get me wrong- They do look horrible to be on the wrong end of. I would shoot any attacker with one immediately.
Also, during the one single fighting knife class I took, I ended up with chalk from my own rubber knife on myself. I was also able to get some chalk from my rubber blade on the other guy (expert), and I obviously had no idea what I was doing. What I learned: It takes some serious practice before the average person can use a blade defensively, and you're probably going to get badly cut even if you practice daily when rolling around with knives because knives are sharp & fighting is unpredictable.
There's also a big difference between being an aggressor & on the defense with any weapon. Aggressor = big advantage.
TL;DR Knives are cool but extremely hard to use defensively, the Civilian especially included.
