It's been a year, but I finally wrote up my critique of "longtermism" (of the Bostrom / Toby Ord variety) in some detail. I explain why this ideology could be extremely dangerous -- a claim that, it seems, some others in the community have picked up on recently (which is very encouraging). The book is on Medium here and PDF/EPUB versions can be downloaded here.
I upvoted Phil's post, despite agreeing with almost all of AlexHT's response to EdoArad above. This is because I want to encourage good faith critiques, even those which I judge to contain serious flaws. And while there were elements of Phil's book that read to me more like attempts at mood affiliation than serious engagement with his interlocutor's views (e.g. 'look at these weird things that Nick Bostrom said once!'), on the whole I felt that there was enough effort at engagement that I was glad Phil took the time to write up his concerns.
Two aspects of the book that I interpreted somewhat differently than Alex:
I agree with Alex that the book was not clear on these points (among others), and I attribute our different readings to that lack of clarity. I'd certainly be happy to hear Phil's take.
I have a couple of other thoughts that I will add in a separate comment.