This is so interesting because in 40K they, the UM, appear very rigid and only work within the confines of the codex. That is the problems that Ventris had in his series....he was a free thinker and thought out side the box or in this case the codex..lol
I never liked the idea of the codex being some rigid rule book. "If the enemy is X strong and placed in Y way in Z terrain, then with a force of A do plan B until C happens." I think a lot of people--some of the authors included--believe it's some sort of play book a third-rate coach would have in a binder.
It's the combined tactical knowledge of hundreds, maybe thousands of civilizations, coupled with the skills and knowledge of possibly the greatest strategist in the galaxy, who had centuries of battle experience spanning the entire galaxy.
I would like to think the UMs are (supposed to be) more disciplined than rigid. The discipline and nerve to do what the situation demands. That might be sitting in a trench for 6 months, to a charge across an open field, or perform to a tactical withdrawal.
Other Legions are, I think, at least during the GC, a bit more rigid. White Scars will relying on lighting assaults and flank attacks with highly mobile units. The Alpha Legion will rely on subterfuge, and so on.
Maybe rigid isn't the right word. Perhaps...less versatile? Not saying the White Scars couldn't hold a line or Alpha Legion take part of a frontal assault, but their focus on particular sorts of combats limit their other abilities and thus their tactical options.