Do you have examples? I am studying philosophy and most people in the department consider it a good resource even when the author of a particular article disagrees with them (in their other published work).
But entry about Colonialism has huge propaganda from Lenin, while not much about positive impact of colonialism.
It has more to do with political economy than dialectical materialism. If you want that, you should read Hegel.
http://plato.stanford.edu/entries/marx/
It seems that SEP's policy is to have articles on either people or topics, but not topics named after people.
SEP has articles on a number of other people who took Marxism and ran with it in different directions, such as Georg Lukács, Herbert Marcuse, and Jacques Derrida, as well as critical theory as a whole. It also has a bunch of other articles on strands of feminism that were deeply influenced by Marx. It is the mark of an important thinker that he shows up all over the place in all sorts of topics.
Whatever you think Marxism is about, I'm pretty sure you can learn about it on SEP. But you can't do that by reading a single article, any more than you can learn a major web framework by reading a single file.
But I do admit that SEP would benefit from more hyperlinks to make related articles more discoverable.
http://plato.stanford.edu/guidelines.html
Articles are supposed to be on issues rather than people with exceptions for historical articles.
The article on colonialism looks pretty good to me, with relativily broad coverage and citations, but it isn't my area of intrest and I don't really have time to read it properly.