Check the datestamps from the forum posts. I’ll bet the ones that said nest integration was “in the works” are from before March 2015, and the ones that said “they have too many restrictions, we probably won’t do it” are from April 2015 or later. Basically the more information smartthings got, the more they found out the issues weren’t technical but philosophical.
You can use nest products with smartthings, you just have to look for the community integrations.
As far as the incompatibility issues with zigbee, @garyd9 is right that zwave makes a much bigger deal out of interoperability for devices from different brands than Zigbee does. That’s actually considered a feature from the zigbee point of view because many security firms use zigbee devices and they like being able to put their own encryption layers on top of them. It makes the networks a little harder to hack.
But Zigbee knows if they want to play in the home automation space for DIY devices they need to address interoperability.
I’ve already mentioned Zwave plus as the newest generation of Zwave with a lot of improvements.
Zigbee is working On their own next generation, zigbee 3.0, and its improvements will mostly be in the areas where it was weak relative to Zwave. It’s going to have much better interoperability, it’s going to combine all its many profiles into one, and it’s going to have a much stronger certification process.
However, for technical reasons it will not be backwards-compatible.
But the fact is if you shop device by device, all of that becomes pretty much irrelevant. You choose the device that works best for your particular use case.
For example there is a Z wave four button key fob, and there is a zigbee three button key fob. The zwave one has been officially tested for smartthings compatibility. It works OK, but it’s a little expensive, the form factor isn’t quite right, and it’s just not a very popular device.
The Zigbee one is a cheap little key fob. Tiny buttons about the size of a car remote. But it’s really cheap, $15. Not officially tested, but community members wrote a device handler for it, and it seems to work just fine with smartthings. Very popular with people looking for a cheap button device, particularly to disarm a siren.
Would I have predicted in advance that there would be an open protocol zigbee device for that purpose? Nope: most of these key fob type devices are associated with a specific security system and the Zigbee ones are double or triple layer encrypted and don’t work with other brands. But one company made a vanilla flavor and vanilla flavor works fine with SmartThings. So you just shop around and see what you find.
(Technically SmartThings uses the basic version of the zigbee 1.2 Home Automation profile, but you still have to check each device for compatibility because of the encryption layer issue. )
Also note that zwave is a little less interoperable then Gary’s statement implies: there’s a basic command set which is just turn on/turn off, and it’s true that any Z wave certified device will be able to accept that command set. But when it comes to advanced features, scene control, or manufacturer specific features you can still find that some features work with some controllers and some work with others. So it’s always good to buy from a place that has a good return policy and to research in the forums before you make the buy to begin with.