I mostly use Siri for home automation, and have a different take.
I have Google Assistant on Nest Hubs—I gave up on Alexa as the spam ("by the way, did you know that…") was super annoying) as well and I don't think Siri is really that much behind.
First off, Shortcuts is WAY more powerful than Alexa or Google's automation platforms, once you learn how to use it. There are far more triggers available, for instance, and you can do more complex if/then and even webhook/API requests which unlocks some really powerful stuff. The app Controller for HomeKit even lets you create automations with more complex trigger conditions, and Pushcut lets you run an automation server for requests that are app-based. Google's Home app is frankly abysmal, Amazon's is cluttered and hard to use.
Google Assistant often thinks I'm talking to it when I'm not, butting into conversations out of nowhere, and often just says "here's what I found on the web" and shows Google results—on a verryyyy slow and glitchy browser. But that's just for random trivia stuff, which I rarely use any of them for—that was fun for like a day when I first got them, but I'm happy to just search the web when I'm interested in a topic.
For home automation, Google and Alexa both say they have "works with Google" integrations, but often times they're quite useless or buggy compared to HomeKit. For instance, my security system "works with Google" but just shows a "no response" on the display; some lights also do the same; sometimes it tries to connect the integration, then it just quits the flow with no further information.
On the other hand, fewer things support HomeKit natively (Matter will help with this), but Homebridge running on a Raspberry Pi has been transformative in terms of bringing everything into HomeKit. (Another good one is the Starling Hub which brings all Nest cameras/devices into HomeKit). The Apple Home app is also the best of the 3 in terms of usability. (Home Assistant is great too, but way more complicated to set up).
They all work fine for me for basic things like setting timers and turning on lights, Siri rarely misunderstands me, and the voice recognition has improved over time from my perspective. It is annoying when you get the "I'm having trouble…" on the HomePods and hope that improves, but it is often down to a networking issue I've found. Upgrading and configuring my router properly has helped a lot.
Also, the Siri voices are the best-sounding of the lot. Alexa's voice is grating to my ears, Google's are OK, but Siri has the most natural and pleasant-sounding of the lot, IMO.
I must be weird cause I rarely use Siri on my phone except while driving to respond to texts; I'm more likely to use it on my watch, but just for little things like "add this to my shopping list" or "set a timer for x minutes" and the raise-and-speak feature is super nifty and usually gets things right.
TL;DR: I'm sincerely curious what y'all wish Siri could do that it doesn't, or where it falls behind GA or Alexa, which are both now being let to rot by their owners since they're just financial drags on them. Siri has plenty of room for improvement, but it does the job for what I need (home automation, timers, alarms, replying to texts).