I have one and it is the least used between HomePod (sounds better with a small footprint) and Alexa (more skills). Alexa is used the most because it is cheap and in every room.
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You hit on a lot of the differences, but the fact that Siri is built into iOS, CarPlay, WatchOS, and Mac gives it a much larger platform than the Dot. I know there are apps that give you some of this type of compatibility, but they don't compare to just saying "Hey Siri".
For home use, the other major differences are:
Echo:
1. Some have Bluetooth in and out (not simultaneously)
2. Some have aux out
HomePod
1. Airplay 2 integrated into the Control Center on iOS and iTunes
2. Voice Control of Airplay2 devices
3. Superior sound quality in a very small footprint
I must admit I was focused on home use in my post, but the truth is I use Siri extensively on my phone and in the car. It has worked very well for audio controls (like skipping over ads in podcasts), hands-free text reading and replying, and on the fly navigation. None of that is even possible with Alexa.
Otherwise I have no point of comparison for the relative utility of Google Assistant on an Android phone.