These services cannot be fully trusted, because they still give power to companies to spy on you.
Why is it not secure?
are proprietary software coming from
Google, Microsoft, Apple, and most eminent NSA collaborators.
you each time before entering your password (which is impracticable),
you leave the email service provider the possibility to retreive your private password at any moment, without you even necessarily knowing it
(since you don't check it).
There is allready the example of the company Hushmail, who gave decrypted emails to the fed, thus proving that they had the power to do it.