From quick reading it appears that a denial can come only from 1) Need to use less expensive drugs first (step therapy) - which is clearly documented in the contract or 2) Because the plan covers only 30 per month, etc., and not more, or 3) Because prior authorization wasn’t obtained for certain drugs (this is the area where they may have some discretion — it isn’t a factor for common drugs).
Regarding insurance in general, claim denials are very often in error, though, and never in the policyholder’s favor. A certain amount of people will just pay, right or wrong, out of frustration with the bureaucracy.