Privilege escalation (e.g. setuid), world-readable files might contain sensitive data, world-writeable files, unrestricted network access (including access to all locally running services)... If you have fully patched system without zero-days and it's configured in a perfect way, then, sure...

Container is quite like a "separate user" except you can explicitly define what it can access.

(Even if all your daemons have good auth, it's now quite common for _apps_ to open listening sockets without much auth...)