I have a Ring security system. The unit has a built-in backup battery and cellular backup when the wifi is down. Its mains powered repeaters also have a battery backup, and the security devices (locks and sensors) are all battery powered. I have had the system for a few years now and never had to change a battery yet.
Security systems should have built-in redundancies that Hubitat doesn’t as it is not sold as a security system. That said, Hubitat is much, much more capable than Ring when it comes to home automation. Ring / Alexa can be used to do some very basic home automations, whereas the sky’s the limit with Hubitat!
For me, having both is the right solution. I feel that I have a good enough security system with Ring, and a really great home automation system with Hubitat. I find that both work together and complement each other very well.
If you want your Zigbee repeaters not to fail with a power failure, you could always put them on a UPS, or purchase USB dongle Tuya repeaters (they have them on AliExpress - I have lots of them and they work great!). The repeaters could be put on a USB battery backup device. It would not look as good and would be much more expensive, but would provide some level of backup.
Remember however that if Wifi fails, the owner would not be notified of an event unless they have some sort of cellular backup for their Wifi as well.