I was stumbling across the same question while i wanted to move away with our files from Microsoft to a provider in Germany.
However the best guarantee will not help if there is still a “man in the middle” having access to the files directly.
You can get as much guarantee as you want, but once the data are compromized, the guarantee will not help to revert it. There is always the human factor in between.
One of the reasons why i am using now my own cloud at home, knowing about the additional risks on it.
Or you encrypt the files before upload. Doesn’t prevent you from stolen data, but it’s harder to use them.