On the eve of a similar question was asked and the head of the US State Department Mike Pompeo. According to Pompeo, Patriot air defense systems "work successfully, but not always."
US Secretary of State:
And magnificent complexes do not allow to shoot down all targets. Even Patriot can do this with varying degrees of success.
The US has already criticized the official’s statement, noting that such a response by Pompeo “inflicts a reputation blow that is even greater than after the“ silence ”of the air defense during a strike at a refinery in Saudi Arabia.”
In the meantime, US military experts suggest sorting out how the Saudi military exploits weapons received from the US. In particular, they recalled an episode when, during a missile attack from Yemen on Riyadh, one of the Patriot anti-aircraft missiles, describing a parabola, fell and exploded a hundred meters from the launch site. So far, the Saudi authorities have not provided data on the number of deaths and injuries from a failed launch, nor on the reasons for the failure when trying to intercept a Husite missile.
Earlier, “VO” published material that Riyadh turned to South Korea with a request to help modernize its air defense system. In the US media, such a request by the Saudis to the Koreans was very jealous, noting that "US specialists could help in modernizing."
As a result, it should be noted that an interesting situation is developing: in the United States, some experts are trying to say that the Saudi military is to blame for the unsatisfactory use of air defense systems, and in Riyadh they make it clear that American systems are far from reliable.