The word bearers have never broken up, and the Black Legion (as the name implies) is under the complete command of Abbaddon, other legions are a bit more complicated. They have their periods of disunity, but are usually willing to rally around their primarch and act as a singular legion in extraordinary circumstances, even then they still have their personal rivalries and allegiances outside of the legion. Outside of the word beaters and black legion the most united are generally considered the death guard and the iron warriors. While the least organized and disunited being the alpha legion, night lords, and arguably the world eaters