What I very dislike about current browser automation tools is that they all use TCP for connecting the browser with the manager program. This means that, unlike for UNIX domain sockets, filesystem permissions (user/group restrictions) cannot be used to protect the TCP socket, which opens the browser automation ecosystem to many attacks where 127.0.0.1 cannot be trusted (untrusted users on a shared host).
I have yet to see a browser automation tool that does not use localhost bound TCP sockets. Apart from that, most tools do not offer strong authentication -- a browser is spawned and it listens on a socket and when the controlling application connects to the browser management socket, no authentication is required by default, which creates hidden vulnerabilites.
While browser sessions may only be controlled by knowing their random UUIDs, creating new sessions is usually possible to anyone on 127.0.0.1.
I don't know really, it's quite possible I'm just spreading lies here, please correct me and expand on this topic a bit.