The Pebble app was removed from the App store, so you have to manually sideload it every 7 days.

And:

Here are the things that are harder or impossible for 3rd party smartwatches (ie non Apple Watches) to do on iPhone:

• There’s no way for a smartwatch to send text messages or iMessages.
• You can’t reply to notifications or take ‘actions’ like marking something as done.
• It’s very difficult to enable other iOS apps to work with Pebble. Basically iOS does not have the concept of ‘interprocess communication’(IPC) like on Android. What we did before was publish an SDK that other apps (like Strava) could integrate to make their own BLE connection to Pebble. It was a clunky quasi-solution that other apps didn’t like, because it was hard to test (among other things)
• If you (accidentally) close our iOS app, then your watch can’t talk to app or internet
• Impossible for watch to detect if you are using your phone, so your watch will buzz and display a notification even if you are staring at your iPhone
• You can’t easily side load apps onto an iPhone. That means we have to publish the app on the iPhone appstore. This is a gigantic pain because Apple. Every update comes with the risk that a random app reviewer could make up some BS excuse and block the update.
• Because of iOS Appstore rules, it would be hard for us to enable 3rd party watchface/app developers to charge for their work (ie we can’t easily make an appstore within our app)
• Getting a Javascript engine to run in PebbleOS forced us to go through many hoops due to iOS — creating a compiler inside the Pebble iPhone app that in itself needed to be written in (cross-compiled to) JS to work with Apple’s restriction on downloadable code can only be JS
• As a Pebble watch/app developer, using the iOS app as relay to the watch sucks since the “developer mode” terminates every few minutes
  https://ericmigi.com/blog/apple-restricts-pebble-from-being-awesome-with-iphones