I agree that there is not really much "planning" necessary to use crimson slaughter. The big differences for most lists are no black mace, no axe of blind fury, and add divination to a sorcerer if taken. I personally will be using it as my new default rather than the codex as the nurgle lord purely benefits from the heart of daemon and a divination sorcerer is a huge buff to the overall low BS chaos shooting and almost every other power helps some aspect that CSM codex needs help with.
The fear is a nice free buff that will be useful against any non SM or CD armies (or nids in synapse). Interestingly enough two of the strongest and most popular armies are both susceptible to fear and fear can be that extra edge that helps out. A great example is a riptide who is normally boss in close combat due to the high T and S coupled with great armour. Throw in fear and suddenly he is significantly worse in CC hitting on 5+ and getting hit on a 3+ is not fun. The funniest part is when your rhino causes fear to the unit charging it.
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