The phrase "unreliable service" means that something, like a product or a person, cannot be trusted to perform consistently or to meet expectations. Full definition
«With Londoners facing the most unreliable services in over a decade, the excuses are wearing incredibly thin. (cityhalllabour.org)
While I had set up forwarding to my parents» house, our campus post office, like many, is notorious for unreliable service. (magnifymoney.com)
Yet PVL commuters pay essentially the same fare as Hudson Line commuters for the privilege of riding the most unreliable service in the entire Metro - North system. (rocklandtimes.com)