I think that the main thing to keep in mind here is this:
MOST PEOPLE, take one of the following mentalities towards their codex:
1) My codex is really strong and it can beat anything
2) My codex got gyped and it isn't as good as your codex, so this is unfair.
Some one saying their codex is a strong codex with a few major weaknesses, is really really rare - or at least saying that after a game in which they win or loose.
To tell the truth, playing vanilla marines, I tend to take the second mentality, even though that's not completely true. One of my frineds plays
SW, and I tried to tell him his wolfstar might have some trouble against
DE.

Apprently
SWs are invincible.
What I am saying is this, everyone has their biases and assumptions, and for some reason, a lot of people feel the need to take and extreme, while normally, the truth is somewhere in the middle. However, trying to tell people that normally makes people angry.