No operating system can protect your machine if it is left unlocked - even allowing physical access makes you vulnerable. A worthy concern, but not in itself an "Ubuntu problem".
most you can get to on a normal unlocked linux machine in normal use is the see browsing history.
you cant even copy files to a usb stick because mounting it requires a password.
that is very different to making the machine yours via remote access.
and very very different than letting browser plugins create user accounts that can be accessed remotely (that have root access by default).
then theres the fact that
selinux seems to be a right state on ubuntu
plus what everybody else said. basically put there are several nicer and more secure distributions of linux i would choose before ubuntu.
you cant even copy files to a usb stick because mounting it requires a password.
that is very different to making the machine yours via remote access.
and very very different than letting browser plugins create user accounts that can be accessed remotely (that have root access by default). then theres the fact that selinux seems to be a right state on ubuntu
plus what everybody else said. basically put there are several nicer and more secure distributions of linux i would choose before ubuntu.