That is incorrect. If you have enabled proximity and background refresh enabled for the Pebble app, then it will not be "killed".
The Pebble App is a user level app hmm, it is not part of the OS. That tells you everything. It is not bad, but you do have significative restriction to its integration with the OS. Since it has a quite limited set of features; this limitation is not too bad. But, for a fully featured Android Wear watch, you will not doing all you could with it.
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Let me provide some insight on this.
I picked up a Pebble watch ($80) 4 weeks ago just to test out the smart watch idea before I decided on whether or not to get the Apple watch.
The Pebble watch relies on it's iOS Pebble app in order to push notifications from the phone to the watch.
Since the Pebble watch app is not flagged as a system process it will and can be killed by iOS if memory is needed.
I'm almost certain the Apple watch processes that will be required to run on an iPhone will be treated as a system process (like phone, music, mail) and will have immunity to being terminated.
Something to consider.
You also have a restricted view of the Phone API (not getting access to the private API) and many multitasking tasks can't be done by user level app in IOS. For people, which has a restricted set of features, I think users won't feel they're losing out too much on the Iphone. But, for a full feature smart Watch, they won't be getting the full effect