“DoH means that Firefox will concentrate all DNS traffic on Cloudflare, and they send traffic from all their users to one entity. So what does that mean? It means people outside the US can now be fully tracked by US government: now some of you might wonder if this is actually in line with GDPR (The EU General Data Protection Regulation). It is indeed very questionable if DoH is rolled out as default, since users do NOT opt in, but have to opt out.”
Check this out: https://www.reddit.com/r/privacy/comments/41cb4k/be_careful_with_cloudflare/