Definitely don't want SPOF. Firefox has both soft-fail and hard-fail modes.. for a soft fail it will fallback to traditional port 53 DNS. Its likely that will be the most common deployment - you need it to deal with captive portals and other split horizon issues as well cloud uptime incidents. But there is a hard fail mode if that is suitable for your environment.

and of course defaults matter a lot, but you will be able to select your preferred DoH endpoint (or not use it at all). Firefox wouldn't lock something like that down.