My home router is running a (regular, port 53) DNS server that blocks requests to ads, scams, malware, etc. I have rules set up on the router so any port 53 traffic that tries to go to the public internet gets redirected to my router's DNS server.

A device on my network that decides to use DoH without my knowledge or consent gets to bypass all that. I can try to block a list of the DoH providers I know of, but I'm not going to get them all. And it's just regular HTTPS traffic on port 443, with nothing to distinguish it from someone accessing a website.