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?
ThePirateBay, the most notorious site in the world, uses Cloudflare. This isn’t China. Wiretapping is illegal in most circumstances, and that’s essentially what it would be doing.