Siri has a hard time understanding English speakers who mumble or have an accent, so imagine how hard it is for Siri to understand someone to whom English is a second language.
On second thought don't imagine it- watch these videos full of Japanese people trying to talk to Siri in English and you'll see how frustrating it can be!
These candid social experiment videos shared by YouTuber NeKo JGT are from the late night variety show Otona No Kiss Eigo, which apparently translates to Adult's Kiss English? Okay then! Here's part 2: