I have set timers with Siri 100's of times. It's practically the only thing I do with it. Hasn't failed me once.
Just yesterday, though, I said "Hey, Siri, navigate home" and that literally got into a failed state. The first time I did it it told me I didn't have a "Home" contact, so I made it. Didn't work when I was in the car yesterday, but it did just now. So, I agree, there are issues with Siri.