At least some of the disagreement shrinks when you clarify "greatest risk" :
Is it a problem for rich first-worlders (dysgenics, maybe), poor first-worlders (immigration), poor third-worlders (global warming, pollution)?
Is it a likely but "mild" problem (dysgenics, sexual immorality, growing inequality), or an unlikely/uncertain but catastrophic problem (AI, grey goo) ?
There's probably still a lot of actual disagreement about facts left (for example, how likely is AI, whether God punishes/rewards us), but I think the bulk of the "disagreement" boils down to "bad for different people" and "bad in different ways".
If I were to ask the question "What threat poses the greatest risk to society/humanity?" to several communities I would expect to get some answers that follow a predictable trend:
If I asked the question on an HBD blog I'd probably get one of the answers demographic disaster/dysgenics/immigration.
If I asked the question to a bunch of environmentalists they'd probably say global warming or pollution.
If I asked the question on a leftist blog I might get the answer: growing inequality/exploitation of workers.
If I asked the question to Catholic bishops they might say abortion/sexual immorality.
And if I were to ask the question on LessWrong (which is heavily populated by Computer scientists and programmers) many would respond with unfriendly AI.
One of these groups might be right, I don't know. However I would treat all of their claims with caution.
Edit: This may not be a bad from thing from an instrumental rationality perspective. If you think that the problem you're working on is really important then you're more likely to put a good effort into solving it.