They're not hard to read until you see the queries that coworkers at large companies tend to generate. If the query is a novella that joins half of humankind's know data sources, the query plan tends to not be much better. Then there's always the dreaded question: "why is it slow?" "Because you're making the database yearn for the sweet release of death."

They're harder to completely understand than they are to read; it often happens for example that SQLite won't use existing indexes, for no obvious reason.

They might mean the output from EXPLAIN which is definitely hard to read in sqlite as it shows you the transaction bytecode.