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).
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.
There is, however, a long article about Marx, and articles about other Marxists. So I don't think your complaint about "embarrassing" subjects is accurate.
No, biased in the sense that it's biased. jkot lists two examples, which are in fact biased in opposite directions. (That might make the overall encyclopedia unbiased, but the articles are still biased.)
No, it totally ignores my world view. Marxism-Leninism was the biggest event in philosophy, since Aristotle and I think it deserves at least small chapter in "authoritative" encyclopedia :-)
If that’s the case, it assumes that the topic named after a person can be learned by learning about the person. However, this is not true in many cases where the topic arose later than the person itself (like, for instance, Christianity), or where the person explicitly disclaimed the topic from having anything to do with themselves (Marx himself famously said that he was not a Marxist).
In cases like that, it is often much more helpful to discuss the actual substance(s) of the topic(s), such as equality, property ownership, markets, exploitation, and philosophy of history (all of which have separate articles in SEP) than to try to dissect the incredibly broad, vague, and diverse set of ideologies that "Marxism" is.
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.
The policy is actually to have articles on issues in philosophy (rather than the philosophies of particular people) with exceptions for historical articles about peoples lives. The article on Marx is such an exception and the lack of an article on marxism is a result of the policy.
