One reason I know they have been against it is because it turned into a political movement that used violence, especially in Central/South America. I think part of it involves the idea of collective salvation rather that Jesus saving each of us individually. The idea that your salvation & my salvation depends on everybody being saved. Kind of like Jesus came to save all of us, of course, but unless we all go, none of us get there, does that make sense? It is unbiblical.