As @techraf and @josef commented, all DDoS providers have the same challenges:
- To effectively identify DDoS traffic, a mitigation provider needs to be connected inside your encryption termination point, or at least be a Man In the Middle.
Your concern regarding CloudFlare is the same concern you should have with them all. If you want DDoS mitigation, you need to let the mitigation provider have access. So start considering what the risks actually are from this. And what you need to do in terms of risk mitigation - perhaps contractual terms with the provider?