New iOS10 and macOS Sierra has a feature called universal clipboard. You can copy something (text, image) on one device and paste it on another. How secure is it and does that mean that everything that you'll copy on any device will automatically upload to iCloud?
Personally, I would not put anything truly sensitive there.
Even if you trust that Apple really encrypts its databases, doesn't give away keys or back doors, the algorithm hasn't been perverted by the NSA, the crypto library isn't buggy, it's also secured in transport with perfect forward secrecy and mutual authentication (no MiTM SSL inspection), RAM is randomized and encrypted, you were not on Wi-Fi/Bluetooth, no renegotiation or compression type garbage... then you still need to consider that every time there is a jailbreak, that means someone has discovered a bug that allows them to completely compromise the security of the endpoint's iOS.
It's another tradeoff of security for convenience. Use it wisely. Consider the risk and manage it. For some uses, it's secure enough. For others, it's not.