Regardless of whether or not you provide your own SSL certificates, cloudflare still uses their own between their servers and client browsers. So any SSL encrypted traffic is unencrypted at their end before being re-encrypted with your certificate. How can such an entity be trusted?
Well yeah, it’s not easy. Which is why they limit what they do to the aggregated data or to targeted discovery.
But that’s only a small technical hurdle and the speed with which you can analyze the data grows much faster than the volume (especially if you are smart about what data you analyze and how you do it) so it won’t last forever.