It is worth noting that this is probably the wrong approach. You will get better redundancy, or robustness, by using a single system - yes, that does open you up to a systematic failure (eg if ZWave had a major problem then your system would go down) but if you use technology like ZWave or ZigBee that have been around for many years, such a systematic failure is unlikely.
Also, I guess redundancy isn’t really what you have, as you probably don’t have every light using both ZWave and WiFi such that if one system failed, you can still switch a light with the other?
The point though is that if you use more devices in a mesh network, you will have a more reliable system - the mesh will be better, the controller will have more options for routing, and overall reliability will be better. By using multiple systems, you potentially significantly reduce the robustness of your system.
WiFi has many issues in this space…
Currently, most WiFi systems are not mesh networks but are point to point connections.
You must ensure that you have a system that can handle enough devices. Most low cost WiFi systems that most people have at home do not support the number of connections required for a “full house” home automation system where every device is on the network.
WiFi will typically not work well for battery devices - ZigBee/ZWave mesh systems are designed with super low power mesh networking in mind.
Also note that there are currently limited application standards over WiFi, so devices can’t talk to each other (unless they are the same manufacturer, but then you’re back to the systematic failure issue).
ZigBee/ZWave all have well defined application layer interfaces to allow devices to communicate and interoperate. You know that if you buy a certified device in one of these systems, it will talk to other devices - even from different manufacturers (in at least 99% of cases anyway ). WiFi is just a transport layer, so you need something on top of this to provide the application - possibly MQTT is becoming more common that it might one day meet this need - who knows.