Are you doing it wrong? I set my alarm every night with Siri in my 4-year old iPhone 6 and I can't recall ever having an issue...
In 2018, the only two ways you can do something that simple wrong, is by 1) not speaking English at all or 2) say something else completely.
Neither, nor. The loading bar or what that is called that appears after the text is recognized but it’s waiting for a response just never finishes. And yes, I’m online.
Siri is just awful and there is just no execuse any longer.