If necessary I can SSH in (if the pi is on) and troubleshoot. No one runs a business out of my home so Internet isn't super critical, and the smartphones have data connectivity if need be. I have a couple fresh SD cards in storage and take quarterly SD card images, so if it fails I don't have to rush out and buy anything. If the pi itself blows up, I can talk my wife through changing the DNS server on the router config page from her laptop, or she can just wait until I get home. Not a huge hassle.

Even so, normally you can set a fallback DNS server on your router, and you could have two little Pi Zeros on a plugstrip in a closet somewhere! (Or just set the fallback at opendns or something)

FWIW, I ran Pi-Hole on a RPi for about six months, and it was flawless (long enough for me to forget the root password :p).

One option would be to set up two Pis, and distribute both IPs in DHCP. If one is down, or can't resolve, clients will fall back to the other one. The chance of both being down at once (unless it's something that takes your network down) seems low, and the cost of a Pi Zero W (~$10) wouldn't add much.

How do your manage your router failing? Do you have a second one in hot standby? Sure, the pi would add a second point of failure but personally I never had a router or rpi fail, so I think the advantage of running a pi hole outweighs that increased risk.