I make some Zigbee devices. I just want to share what I know based on what I learn in working with Zigbee devices. I am sorry it is rather long since I want to give you some context of how a problem can arise due to power outage. I do not know anything about z-wave.
Your Zigbee network may consist of Zigbee Router and End devices. The Hubitat hub is a Zigbee coordinator.
Zigbee router device is typically devices that is powered by main power(your AC outlet. switches, and some bulbs). These devices can be a parent to Zigbee end device. I make one of these router modules too.
Zigbee sleepy end devices is all your battery powered devices like contact sensor, leak sensor and etc. This type of device require a parent to work. The parent is either your hub (coordinator) or those Zigbee router.
Lets assume that your hub is the only one is running during the power outage because you add battery backed up for it. All your battery end devices will try to find a new parent. Some will find the hub and asked the hub to be its parent. The hub have limit of approximately 20 to 30 something end devices( I do not remember exact number). If you have a lot of sleepy end device, this could be an issue because all those end devices will look to your hub. Some end devices will loose out because the hub is already full.
One my ask, why it is an issue for these orphaned end devices? Shouldn't the sleepy end device keep trying to find a new parent? If it does keep trying, the end device will find a new parent when the power back up. This expectation is not quite correct. Sleepy end devices will eventually give up trying to conserve its battery. That process of scanning for a new parent is expensive in term of power consumption. Some device will try for 20 minutes and give up. Some will try a couple times only.
The same issue is expected for end device that need a router to talk to the hub because it is far from the hub.
After the power recovery, I believe you may need to go around to the impacted end devices. You may need to press some button in some cases. In other case, you may need to power cycle the device. In some extreme case, you may need to re-pair your devices. This issue has nothing to do with the stability of your Zigbee network.
In summary, you should consider to have not only the hub is battery backed up. It would be wise to have some Zigbee routers devices battery backed up. This will minimize the issue that you will be dealing with the aftermath of power outage.