plain sailing vs unchallenging

plain sailing

adj

  • simple, straightforward, or easy 

noun

  • The navigation of waters free from hazards or unfavourable winds. 

  • Something that is simple, straightforward, or easy; something that offers no troubles or difficulties. 

unchallenging

adj

  • Not challenging; easy to do. 

How often have the words plain sailing and unchallenging occurred in a corpus of books? (source: Google Ngram Viewer )