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