Perhaps you find FHI equally non-credible/non-accountable as a charity, but whatever FHI’s failings, it’s hard to see how they are exactly the same ones which you have ascribed to SIAI. Perhaps you believe that if a charity has not been evaluated and endorsed by Givewell, it can’t possibly be worthwhile. I can’t avoid the thought that if you were really interested in existential risk reduction, you would spend at least some tiny percentage of the time you’ve spent writing up these posts against SIAI on investigating FHI as an alternative.
See my response to Jordan’s comment.