Fist up a disclaimer: as a non-native speaker, plese excuse the stale writing.
Well I think folks are dismissing the one, two (or even three) things the Slaangors have going for them.
First up: they have range 2" on all their attacks and come with 40mm bases. That means it fairly easy to get a them to "fight in ranks". That means they will be able to coordinate their charges with other units. E.G its so easy to get 2x 3 slaangors and a chariot to engage the same target at a narrow front.
The second, and in my eyes the most important advantage they have over the bullgors is the way their 2" reach works with the new locus of diversion. If they (the slaangors) engage a unit with reach 1", the fiendbloods can simply not be attacked back because of the "no pile-ins".
Lastly, as a slight bonus, a big herd of Slaangors can deal quite a few mortal wounds to hero hanging near a charget unit. If the Slaangors get a single model within 3" of the hero during charge or pile-in. they can roll for obsessive violence for the whole unit, not just the models within 3" of the hero and allocate the wounds to the hero, while using their attacks to target a different unit.
The Slaangors are a unit that rewards tactical movement and positioning. So I´ll happily give them a go 🙂