There's all sorts of opinions on the Blood Raiders, Sani Sabik, etc. etc. All of them are valid. Below are mine. I should also point out that I haven't read either of the EVE books, which I think feature the Blood Raiders pretty heavily.
That being said, I've always thought the Blood Raiders were primarily a criminal organization with some religious leanings, rather than a religious cult that also engages in criminal activity on the side.
Sacrificing kids, taking slaves, draining ships' crews of blood may have some sort of religious significance for your average Raider, but I'd guess that many of them realize that these (very evil) actions serve practical purposes as well. It is this practicality that forms the basis of their "friendly" relations with other pirate factions.
I think the Raiders' practical orientation has been illustrated pretty well in the PF (that I'm aware of), kidnapping Hnolku and the Insorum attack, Chamberlain Karsoth, etc. That's not to say that the Blood Raiders are criminals in the vein of the Angel Cartel, the Guristas, or the Serpentis, who represent more traditional models of organized crime (although they all probably engage in some pretty frightening and villainous things on the side as well).
When it comes to the Sansha, I can certainly see how their alliance could be very tenuous. If you think about the ideological purists on each side, you have quite a conflict.
Sansha True-Citizen: "We will create a perfect society, no pain, no suffering, utopia. All people have value and purpose in our world. Everyone unite!"
Sani Sabik Cultist: "Lesser beings are undeserving of such a world, why give them anything at all? They exist to feed us."
So, what it must come down to, because both sides are certainly allied today (Master Kuvakei had some very nice things to say about CR-SE, joke's on him haha), is that while it's very easy to characterize the the Raiders as these sort of stereotypical cultist lunatics, they are in fact filled with reasonable and practical people more focused on acquiring money and influence than searching for mystical artifacts to complete ancient rituals or drinking goblets of blood.
This isn't to say that people can't draw pentagrams on the floor of their cargo holds if they want to, but that's not for me.