Smartwatch compatibility One can get a "cheap smartwatch" (most likely running Nucleos) for EUR 30 or less, a Pebble watch (usually running PebbleOS) for EUR 100 or less, or a watch running android-wear starting at a little more than EUR 100. Having decided to check with the first variant, I've seen its use from the point of view of an Android user is quite limited: it makes a lot of noise if it gets too far from the Android device (to prevent you from losing it), and it can show some (not all) notifications. Squinting at the latter, I notice it requires a huge companion app plus the Google services to work with an Android device1. I'm not at all sure about the Pebble, running an OS based on the FreeRTOS kernel.


  So my question, purely from the point-of-view of an Android user: What are the pros/cons and compatibility issues of those systems?


Please leave aside device specific things like water-proofness, Svarovski stones on the corners, or "valuable brandings": I'm not asking for a shopping advice on a specific model here. Also please keep out "personal preferences" (those which fall into the category of "primarily opinion-based") as core element (side-marks are of course OK), but rather stick to facts. What I'm looking for is general/canonical information about compatibility and basic features concerning their interplay/interaction with "my Android device" (you could phrase it "how they improve my Android user-experience").

(Using my Google-Fu on "Pebble versus Wear" etc. mostly brought up specific device comparisons only, like "LG G Watch versus Pebble round" – which is not what I'm after. I don't want a device comparison – I want some background on the underlying systems and their "co-operation" with Android)



PS: With a first answer being available, I'm also specifically interested to know which of the systems works without Google Services being installed – or whether there exist dependencies other than their "controlling app". Backing by "official/authoritative sources" would be very welcome in this context.



1: My devices are running without GApps, and the microG replacement does not yet support Wear
Android Wear - Developed by Google Inc., an official adaptation of the pre-existing Android ecosystem for wearable devices. 

Pros : 


Developed by the same company that owns and contributes to Android,
therefore underlying compatibility and the promise of future updates. 
More apps, due to the Google Play app store. 
Deeper linkage with the Android ecosystem.


Cons :


Requires the usage of proprietary applications that aren't open sourced. 
Alternatives don't (as of now) offer comparatively equal services as the proprietary apps.1




Pebble OS - Developed by Pebble Inc. for sole usage in their products

Pros :


Not limited to a single phone ecosystem
Doesn't require GApps to function properly.2


Cons :


Requires the usage of Pebble proprietary apps.3
No underlying compatibility baked into Android, therefore less capability and integration at a lesser level. 
Lesser apps as compared to Android Wear.  




Nucleus RTOS - Developed by Mentor Graphics. 

Pros :


Supports various target CPU architectures.
Uses the FAT file system.
Doesn't require GApps to function properly.


Cons :


Closed source (source released only to customers)
No underlying compatibility baked into Android
No generalized app store for usage of various applications4
Limited in interaction with Android devices5




Android for watches - Unofficial modifications of Android for usage on smartphones6

Pros :


Access to the entire Google Play app store.
Gives most (if not all) features of Android on a smartphone.
Doesn't require GApps functionality


Cons :


No guaranteed updates.
Modifications aren't open source.
Less or no support/warranty from OEMs.




1: Wear support is baked into Google Play Services, and additionally requires the Android Wear app. With microG there's an open-source alternative to Play Services (see Android without Google: microG), but Wear support is not yet implemented as of 4/2016. It's in-the-works, though, so it might become available in the second half of 2016. Which would mean that currently "Android Wear" smartwatches are not an option for those preferring "Google-free systems" (without GApps). (note by Izzy)

2: According to Pebble support, their proprietary app relies on functionality of GApps. Quoting: *"Yes, the Pebble does use Google services." Nevertheless, I'm successfully using a Pebble Time Steel without GApps, having the FOSS replacement microG installed. (note by Izzy)

3: The pebble app is only available via Google Play Store. But even if you can side-load it from another source, in order to initialize and use your Pebble you are forced to create a Pebble account – via which, according to their TOS, again a lot of data are collected about your usage etc. No opt-out. (Quoting Pebble service: You will need to make a Pebble account using an email address. After you make the account, you can opt-out of sending analytics/diagnostics. ) This will be a show-stopper for the privacy aware, which for exactly that reason maybe even got rid of the GApps on their devices.  (note by Izzy)
As pointed out by Pebble service, you can alternatively use the open-source Gadgetbridge. Using that, you won't need a Pebble account, no data is sent to Pebble – but also no support for it by Pebble itself: *Please note, that if you go this route, we do not have support for it. I successfully initialized my Pebble Time Steel with Gadgetbridge, and also was able to upgrade the firmware and install a watchface. (note by Izzy)*

4: Most of the Nucleus watches don't even support installing apps. They come with a set of pre-installed apps and watch faces, which you rarely can modify. (note by Izzy)

5: Often only support a subset of notifications, e.g. missing to show those coming in via GCM. Some allow to control the Android devices camera – but usually that's all to expect. (note by Izzy)

6: Unofficial modifications are not licensed or recommended by Google, and such watches are now slowly deprecating in favor of Android Wear.