Cant speak for the other person, but as someone who's used iPhones exclusively since the original, Siri makes mistakes with sufficient frequency that it's subjectively just bad. I can't quantify what percentage of requests fail to be understood correctly, but it's sufficiently high that the frequency is annoying. It's even worse when using the Apple Watch (Series 3).
Unexpectedly, this can sometimes be a positive too. "Hey siri, turn my lights <pink/orange/purple/blue/red/white>" has on multiple occasions been interpreted as "Hey siri, turn my lights", which then results in the lights being turned off. This is hugely entertaining when you're demonstrating your new Philips Hue bulbs over FaceTime, and the person on the other end now just sees your camera go black instead.
Unexpectedly, this can sometimes be a positive too. "Hey siri, turn my lights <pink/orange/purple/blue/red/white>" has on multiple occasions been interpreted as "Hey siri, turn my lights", which then results in the lights being turned off. This is hugely entertaining when you're demonstrating your new Philips Hue bulbs over FaceTime, and the person on the other end now just sees your camera go black instead.