This is, obviously, something I've been thinking a lot about both in relation to other critics and more recently regarding myself. Being conscious of our ignorance is a special kind of torture, I think.
One of the appeals of genre criticism to me is that it is a relatively small field. Particularly, I've found myself drawn to fantasy criticism, specifically because there I have the ability to simultaneously narrow my field of reading while still reading deeply. In other words, I can tackle it knowing that I can eventually become knowledgable in the subject... and that is the hallmark of a good critic, isn't it?
So on the one hand, I can eventually find and read and process everything that has been written on the literature of the fantastic.
On the other hand, it's a self-imposed limitation, so I am not obligated to remain ignorant of the broader spectrum of literature and literary theory. I have the freedom to pick and choose and try to discover those relevant texts that will affect the field I choose to work in. Whereas in a broader field, it is almost impossible to keep abreast of what has or is developing in that body of literature. Is, for example, Middlemarch relevant to fantastic literature? No, not particularly, and therefore I don't have to read it if I don't feel like it. But just because I've chosen fantasy as my field doesn't preclude me from reading it.
I think that is what critics need to do; to narrow their field of study sufficiently that they can become knowledgable in that field, because it takes a superhuman to have read everything. We can't all be Michael Dirda or Harold Bloom. But being aware of our limitations goes a long way to making sure that it doesn't become a fatal weakness.