CloudFlare's "Flexible SSL" (https://www.cloudflare.com/ssl/) offers encryption/authentication from CloudFlare's server to the client, but none from the origin server to CloudFlare's. Which means that is a vector by which the content could be sniffed or modified in transit.
It's a "better than nothing" option, as there are a slightly higher number of actively exploited attack vectors that apply to the client to CDN connection than the CDN to origin server, such as "free" wifi that injects ads, malicious ISP DNS, and the like. But it's not actually secure, as the origin server to CDN connection could be tampered with, and just because there are fewer active attacks that would be likely to affect that connection right now, doesn't mean that someone won't come along later and hijack such a connection.
CloudFlare offers other TLS options that do include encryption and authentication between the origin server and CDN, but they do require that you set up a certificate on your server, so if all you're trying to do is enable TLS (and don't care about the CDN), just installing a cert on the origin server and using TLS is probably a simpler option that using CloudFlare.
We encourage users to use Strict mode which requests and validates a certificate from the origin.
It's great that shared web hosting providers and others are starting to make it easy to acquire and install a certificate, but that hasn't always been the case.
EDIT: We also provide an API that will provision a free certificate for your origin: https://blog.cloudflare.com/cloudflare-ca-encryption-origin/. The certificate is optimized for communication with our edge (essentially just as small a chain as possible, as we don't need the intermediate to walk to the root). Either that or use certbot from EFF/Let's Encrypt.
It's a "better than nothing" option, as there are a slightly higher number of actively exploited attack vectors that apply to the client to CDN connection than the CDN to origin server, such as "free" wifi that injects ads, malicious ISP DNS, and the like. But it's not actually secure, as the origin server to CDN connection could be tampered with, and just because there are fewer active attacks that would be likely to affect that connection right now, doesn't mean that someone won't come along later and hijack such a connection.
CloudFlare offers other TLS options that do include encryption and authentication between the origin server and CDN, but they do require that you set up a certificate on your server, so if all you're trying to do is enable TLS (and don't care about the CDN), just installing a cert on the origin server and using TLS is probably a simpler option that using CloudFlare.