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)

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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.

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^{1: My devices are running without GApps, and the microG replacement does not yet support Wear}