Generalist v Specialist Detachments

I'm somewhat new and still in the "collecting a bit of everything/what looks cool" phase, and trying to get a sense of how well each detachment does in terms of requiring investing in specific units or not. As far as I can tell, Needgaard is the most general while Delve is the one that wants you to go heavy into Cthonian units specifically. My question isn't necessarily about which Detachment is best, but how much investment into a specific type of unit is recommended for each.