"What do you mean by "nuclear" here?"
https://www.britannica.com/topic/nuclear-family
"However: I would argue that whether the guidance can be called "sound" depends an awful lot on the number of its followers who fail to follow it correctly, and if that proportion of followers reaches some threshold."
That puts the cart ahead of the horse. And to take the analogy further, if the cart loses a wheel, it's not the fault of the horse that pulls it.
"For example, if say over 40-60% of a religion's followers departed from the sound guidance, there would be a very strong argument for saying that the guidance therefore is not sound at all."
Or that the teachers of that guidance stink, and that is sometimes a problem in Islam and in nondenomination Christianity, because there's no authoritative heirarchy above the teacher to say to that teacher "Excuse me, but have you lost your mind?"
"This is most clearly seen in the Synod of Bishops of the Catholic Church, but applies to Islam too (after all, there are many interpretations of the Quran and Islamic law, and as such the views of Muslims plays a big part in which interpretations prevail). This implies a degree of complicitness, but nonetheless it is not a simple one, as you say."
The "problem" that Islam has in this regard is that there's no central authority. We have certain positions of high respect that one does not ascend to without a large consensus that the candidate is well educated and balanced, and there are several well-accepted schools of interpretation which (largely) concur with one another and which religious rulings are expected to concur, but reality is that any educated cleric can fire off a religious opinion at any time.
"I'm not sure that theists have an issue with anyone who points out that within the religion there are adherents (ostensibly) that have deviant beliefs and/or deviant practices. Some people of course feel the need to circle the wagons when criticism comes from outside, but most readily admit that there are bad actors inside of any faith. (It's kind of obvious, eh?)"
I am not so sure of that. While theists generally accept....Still, that's a complex and difficult discussion.
Extremely complex.
"The aim is to state that religious believers should consider this issue more - but certainly not to ostracize them or similar."
Theists in general prefer not to accuse others of heresy. In Christianity, the general thought since the Reformation has been that it's God's business to determine who is a heretic and who isn't. And in Islam, it's even more complicated. :-)
But, in general, I'd agree. The reticence to hold other believers accountable for variant beliefs has let to many difficulties over time.
