Have a passcode set, and set it to wipe the phone after 10 failed login attempts. With a passcode set you can only access the phone from a computer that has previously accessed it, on any newer computer you need to give the passcode. A very skilled hacker can probably get in through DFU mode, but ordinary mortals cannot - and no available tool will bypass it, as far as I know.
----------
Oh, and SSH is pretty safe, as long as you remember to set roots password and mobiles password. But remember, if you restore and rejailbreak the phone, they are both set back to "alpine", even if you read in from backup.
