I don't have the know how to talk about safety and privacy, but here are some caveats.
I think you have to use their client and can't add your adress to 3rd party clients like thunderbird. Their client is however nice to work with.
If you forgot your password, the only way to change it is by using a key that is given to you after account creation. Keep it safe! Check for spelling errors If you lost the note or it's not in you passwordmanager or whatever you use, your account is not recoverable. Their support can't help you reset your pw.
Other than that they make email encryption pretty easy with a checkbox right under the recipient in the email editor.
Another handy feature are the aliases. (Payed feature) You can set up some email adresses for certain purposes, and filter their traffic into different inboxes quite easily. If one of them.get's compromised, deactivate and move in. Your master adress is probably still usable.
What I do not like is the fact that paying customers get support first.