Specifically, they are considering making CloudFlare the default nameserver. A new feature called "Trusted Recursive Resolver" (TRR) could be turned on by default, and therefore override the DNS changes you've configured in your network.
Cloudflare says it takes your privacy more seriously than telecommunication service providers do because this DNS query will be encrypted, unlike regular DNS. They also promise not to sell your data or engage in user profiling.
This is a deep subject with many, many layers. Dig deep on this one. So, the question is — under what circumstances would it be OK for Cloudflare (or any other third party) to take over our DNS by default?