Cloudflare reserves the right to add adware and spyware to your pages as they pass through Cloudflare's MITM center. ("CloudFlare may: ... Add script to your pages to, for example, add services, Apps, or perform additional performance tracking."[1])
CloudFlare will make it clear whenever a feature will modify your content and, whenever possible, provide you a mechanism to allow you to disable the feature.
When privacy and security require an opt-out, you can be sure the vendor plans to violate them. Their "free HTTPS everywhere" plan has to be monetized somehow.
