I do think it is more difficult to find a critique group for non-fiction. All fiction has a common core: an interest in story. You can still usefully critique a piece that is outside of the genre's you usually read because there is still a story there, or should be. In fact, critiques from people outside of your accustomed genre are often particularly useful because they are not charmed by all of the genre trappings: they only see the story.
It is not like that for non-fiction. There is no common core. You need feedback from people who are actually interested in your field. That is much harder to find in a local critique group. It might be possible in a university town, but even then it may be hard.
I'm not sure how well anonymous critique sites fit this problem either. They may have enough people registered to find a match for your subject, but how do you know that that is who you are getting. How do you know what axe they may have to grind? The other problem is that you will only get feedback from other people who are trying to write books. For non-fiction, particularly, you want feedback from people who are readers or practitioners in the field, not necessarily writers.
I think a better way to get online feedback on non-fiction is to start a blog. That way you can open up your ideas to commentary from the whole community, not just other writers. This certainly worked for my on my last non-fiction book. The added benefit of this is that not only do you get feedback and discussion, you also build an audience for the book, which can in turn help sell the book to a publisher.