You can trust iCloud only as far as you can trust cloud storage. Example: Google uses cloud storage. I use it extensively. If my password is discovered by a would be intruder you would think my stored items would be vulnerable, right? Wrong, Google offers added security called 2-step authentication. I, myself, can't even log into my account from a foreign computer without my phone that has the Google Authenticator app on it that is associated to my account. It displays a different 7 digit code every 60 seconds making a second, random, 7 digit password needed to gain access to my secured storage.
I am not sure that Apple has such features but if they do I would use them because to my knowledge Apple has already lost their entire database of user names, passwords, Apple ID's, unique device ID's and much other personally identifiable information in complete 3 times and in partial many more times.
Bottom line: your cloud storage is only as secure as you are secure! If you trust you, then you can trust it. If you don't know a thing about my comment chances are your cloud is insecure.
You are responsible for your own data security. Google drive and Dropbox are good cloud storage solutions that both offer 2-step authentication. If you sign up for Dropbox with this code: https://db.tt/3D7Vap1e
and they will give me extra storage. I already have 60 gigabytes with them and it's free.