This recent summon update in this firmware got me thinking more... We know that phone key isn't so reliable, especially on android. What's to say that activating summon on the phone app and needing to stop the movement of the car urgently with the app will be 100% reliable? On the S/X, you push the keyfob and the summon stops but there is never a problem with the keyfob....
The issues with the phone-key generally occur because the phone has not yet connected to the car (e.g. due to the Tesla app not running in the background, issues with Bluetooth staying awake, car being in deep sleep, etc). Once that connection is established, it works as expected.
Inasmuch as Summon requires your phone to be awake and already connected to the car before it will work, the factors that cause issues with the phone-key do not (or, rather, should not!) apply. And the general potential pitfalls of Summoning a Tesla via phone (e.g. dropping your device mid-summon, accidentally closing the Tesla app, etc) apply equally to the S/3/X.
Separately, I wonder whether, just as you can stop Summon by touching a door handle on the S/X, you will be able to press a Model 3's handle inward to accomplish the same.
Hopefully we'll get to see some real-world tests from Model 3 owners soon...