Just come across the eggs/prunes bit in a book. Is "to compare eggs with prunes" an idiomatic expression meaning "to juxtapose totally different things", or just a licencia poetica by the author?
Also bear in mind that eggs supposedly give you constipation whereas prunes are well known for their laxative effect. So the author is not only comparing two things that cannot be compared, s/he may be drawing attention to the opposite effects of two substances/situations. Hard to say without the full context.