Its probably not much but these are my 2 cents.
Word Bearers
With the word bearers its good to note 2 things, 1 they where extremely loyal to Lorgar. And 2 they where the 1st legion by a long margin to actually turn traitor, so they had plenty of time to weed out. As more of a side note, they did purge weaker parts of their legion during the battle of Calth and the shadow crusade but that's quite far past Isstvan 5.
Night Lords
The night lords pretty much went rogue as a legion. And I'd suggest that their actual reasoning of going heretic are fairly different than the other legions. It is noted however that in
Iron Warriors
The Iron warriors where quite a split legion. However when word got out that Olympia turned heretic, Perturabo went for a mass assault on the planet millions of civilians where killed. but more notably A part of Iron Warriors resisted the orders from Perturabo seeing how crazy and cruel they where, and they where gunned down by their own battle brothers. Though the legion as a whole was really stunned at this. It quite shows what they would do for their primarch and it probably spiraled do quite fast. which is described in Perturabo's Primarch novel.
Alpha Legion
For the alpha legion I don't quite know, it's complicated. It is said that they where still loyal to the emperor, though with Alpharius disagreeing with Omegon on how to help the imperium by siding with Horus, Alpharius more willing to sacrifice and Omegon trying to do as little damage to the imperium as possible. So I can't fully say what's up with them other than it's complicated....