Well I'm not an RF engineer so I'm not going to try and talk about RF contention or things I'm not sure of.
I can say that in my environment I have lots of controllers and lots of devices in use and many coming and going. I don't have issues with "contention" or any interference between the controllers. I have problems with certain gateways with devices but none of that is RF related.
Yeah.... I've only experience THIS problem on two gateways I've ever used and tested..... This isn't a Z-Wave problem.
I will caveat this... unless you have 50 devices all reporting at 30 seconds this will overload any controller but it's still not a RF problem it's a controller bottleneck of serial communications.
Well yes and no. If Z-Wave is the transport for this information (as it often is) then it can quickly saturate the bandwidth of that network causing it real problems and making it slow or unmanageable. Particularly if the Z-Wave network speed is being reduced because of retries or legacy Z-Wave nodes. It could even be reduced to 9600 baud (a well designed network will be at least 40K+)
Of course the controllers are the ultimate destination of this sensor information so they can get saturated and (as you have edited your post) then true, if you are attached to a serial based controller with it's own speed limits then that can be swamped too. Most credible Z-Wave controllers (I assume) are not limited by a bottleneck slower than the Z-Wave bandwidth so...
It invariably is..
We're both saying the same things BTW
Kevin
RF Engineer 
If this is an RF "problem" then how is it possible to run not just the 232 devices a single controller supports but run within a multi-controller setup with 400+ devices if the RF is going to be saturated? Because the devices aren't saturating the RF frequency (unless you have a very very bad device or multiple bad devices).
Lots of these "problems" get blamed on "network contention" when it's not the limit to the RF frequency but the gateway limitations.
Yes we are saying the very similar things. I'm just trying to explain as I can so as not to scare people away from Z-Wave thinking they are going to limit themselves when the reality is nobody is going to be running several hundred devices ALL communicating at the exact same time to consume all of the RF spectrum at that given time. Lots of "problems" get blamed on "Z-Wave Network" when it's not the network, most of the time it's actually the gateway and software stack that is inefficient.
To agree with your points. It is absolutely possible to saturate the RF for Z-Wave. Is this normal? No, not even under heavily used and large installs.
We mostly agree except that any mainstream controller should be able to handle the incoming Z-Wave traffic at full possible bandwidth without clogging. Z-Wave is after all a slow network. Yes I agree that controllers under resourced in hardware or bad software stacks would contribute their own issues.
It is very possible but , as you also caveat, when devices start consuming the bandwidth with lots of data, have bad transmission paths requiring retries or when the network mesh has issues and may even get restricted down to 9600 baud it gets clogged. You can have 232 devices if they all are sensibly sparing in their data transfer, and yes with more controllers hundreds more. Having 100K of bandwidth with Z-Wave + is helpful too. Above all get rid of 9600 devices.
@didymus Sorry this went OT slightly . I use both technologies and for typical usage either will serve you well. Yes I prefer Zigbee but I don't use it very much - just 10's of devices.. Often your choice will be based on the device you need. Using both is very acceptable although you need more repeaters to build your meshes.
Well it's a lovely day here, with sun that we've had for the last few weeks, destined to end tomorrow so I'm going to catch the last bits.
If you have a house full of plumbing, metal and aquariums, then I would maybe think twice about having z-wave or zigbee. The chances of having radio interference on the z-wave frequency is almost none. The only challenges you will have would be physical obstacles in your home. Metals, water, etc. Technically, you could be in a condo or apartment where maybe another smart home would interfere, but that would be a small number of instances in the United States. I could see over seas where this might be more of an issue.
Just want to point out (as I have in other threads), I believe network crippling RF Interference on Zigbee and Zwave is a lot less frequent that supposed by many who are having problems with their networks.
@kevin's the RF engineer, I happen to have a passing aquaintance with RF, and am certainly not an RF engineer, but I know enough to know that A. Your house is a horrible RF environment in general, and B. The designers of both Zigbee and Zwave built network architectures, radios, and protocols with this in mind.
The occurrence of network crippling interference has to be extremely small, or these protocols wouldn't be functional in the real world. That's not to say we haven't seen it. The guy in the UK that lived right next to a BT Wifi tower, with (no doubt) amplifiers driven well into saturation, and likely poor frequency discipline, is a good example. His near field RF environment was so saturated that he couldn't get Zigbee, or even WiFi to work.
Sadly, without a good spectrum analyzer, and possibly some sort of BER testing on a network, proving RF interference in a home is tough. But keep in mind (again) that Zigbee and Zwave were designed with common household devices in mind, and the ability to COEXIST with other networks on the same frequencies.
Frankly, except in extreme circumstances, Zigbee and Zwave should coexist with just about anything you have in your house. WiFi and Zigbee have been coexisting for years, and as @april.brandt pointed out, there's a lot less potential in-home interferers on Zwave+ frequencies.
Just my idle thoughts to follow up on Kevin and April's points.
S.
I don't believe interference is a big issue on either, however the robustness and therefore speed of your mesh is, especially if you have a lot of devices. Accessible bandwidth is the key to it all working smoothly.
My house was full of plumbing, but then I got rid of it in favor of this:
My zigbee mesh has never been stronger, but for some reason the WAF has never been lower... 
I .....
WANT ONE!
I .... wan't Two
hmm edit: now what use would that be ?
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