That's always going to be true at some level—even if you're hosting DNS yourself. I self-host the DNS for many of my domains on a Linode VPS. If Linode decides

That's always going to be true at some level—even if you're hosting DNS yourself. I self-host the DNS for many of my domains on a Linode VPS. If Linode decides that they don't like me (or they just make a mistake) and re-allocates my IP addresses to someone else, they could set up their own DNS server to direct my domains however they please. However, I trust Linode to not do this, based on their track record and reputation. You're always at the mercy of at least one vendor, unless you own your own block of IP addresses and advertise it via BGP (and even then, someone could make their own malicious advertisement of your IP block). You can't eliminate the risk. At some point, you have to set the threshold for what risk you consider acceptable. For many organizations, using Cloudflare provides enough benefit to outweigh the slightly higher risk of something going wrong.