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?
If you want then to cache your content to reduce the load of your servers, they have to decrypt the traffic. This is how a reverse proxy works.
And, well, you have to trust them before contract their services. The same way people trust vpns to route their traffic. If I was from some 3 letter agency and want to spy on potential illegal content, I would tap into a vpn server.
Or simply fund one and advertise it on youtube.