> prose and an expository essay -it was doing both.

Putting prose in an essay means there are more valid criticisms of a piece of writing, not fewer. If somebody is breakdancing and reciting the periodic table at the same time it’s ok if somebody notices if they skipped the lanthanides and actinides.

I’m a fan of blending the two! It’s just really really hard to do both well at the same time. My most recent example is Malcolm Harris’ history of Palo Alto, it is incredibly well-done.