The numerical estimates, where they exist, are often very rough estimates. Whenever a group commits atrocities, it usually puts real effort into trying to obscure the scope of its crimes.
Focusing on the quantitative information to the exclusion of the qualitative can lose sight of the unusually cruel and horrible things that some people experienced. Mass atrocities can often be described as 'uniquely bad' in their own way, in that people experienced things that were rare and/or notable. Qualitative information really can't be ignored.
When attributing deaths to political systems, such as communism versus capitalism, I feel that a reader's/writer's own political lens must inevitably distort the analysis. I think it may be impossible to do this task objectively.
Note that the book was written by a layman, someone with little education as a historian.
Read carefully the section titled "Academic analysis". Overall, I feel like there's cautious support for his book, however, there are some important caveats.