As safe as they can get. You could never be 100% sure of course. They designed their whole OS for shared hosting, and one of the features is the jailed envoirment: CageFS. The system keeps one copy of all system files which every user is going to see (they can't be changed either by users). All users have their own set of files in /etc/ like /etc/passwd with the system users + only their own user. This jail works for ssh and apache, php and cgi etc.
They have a free trial so you could test it all out if you want. You could just login into ssh as a user and check all places you can go. You might find something that needs to be hidden or added. They have a pretty good support team if you need help with that.