I am in favor of separating the large fact tables into their own filegroups - based on better data and analysis.

I don't necessarily think that the fact tables should each have their own filegroup. But if you can split out the fact tables into some configuration of new filegroups - it would prove beneficial.

Where is the benefit of doing that?

With data this size, it provides a more robust opportunity for recovery. Sure you can see some performance gain (whether on same disks or not). But the big benefit is being able to take a filegroup backup and restore a filegroup backup (when/if necessary).

Maybe somebody wants/needs a 2TB group of tables restored to a dev or qa environment. With FG backups, you can achieve that without having to concoct a process to move the tables via bcp or ssis, or worse via backup/restore of the entire 50TB database.