As a user whose trust has already been broken by both ISPs and governments, I see no drawback in participating as a user in this public experiment. What you describe as a drawback is a privacy improvement for me.

The DNS implementation used by every non-Tor user around the world today is already subject to warrantless spying by every ISP and government in the world, due to the property known as “cleartext”. If you opt-in to the Cloudflare trial, you are only at risk of warrantless spying by Cloudflare — rather than every ISP — and the US government — rather than every government.

My cellular ISP sells my DNS queries to advertising networks, and my home ISP is wiretapped warrantlessly by the US government. This experiment decreases the chances of the resale of my personalized data to data warehouses and decreases the chances of success of warrantless wiretapping by my government.

I envy those of you that believe you can trust your ISPs and governments.