I personally oppose the limited-scope memberships. If our aim is to write well, we need to take advantage of any opportunity to improve.
Even more important, novelists need to learn to write narrative passages, which is a common weakness in fiction; nonfiction writers need to figure out how to write anecdotes and good stories.
Most important to me is that we can always glean and grow from other writers. Too many fiction writers in editing groups begin to sound like each other. By editing outside their area, members gain information that they probably wouldn’t otherwise read.
Good writers seek opportunities to grow.
Regardless of the genre, they can learn from each other.