Care to elaborate? Does FreeBSD make assumptions about the root user's shell that Linux doesn't?

Yes, root's shell should be one provided by the distribution's base install.

I think the biggest worry is that if you need to do something like boot into single user mode for an emergency recovery, and /usr or /usr/local (with root's shell in it) is on a different or unmountable partition then you will not be able to do anything.

Additionally, if you upgrade your shell, or something gets borked, then... you now don't have a shell.

This is one of the reasons for the existence of the 'toor' user, which still uses the default shell

Say there's a failure that forces you to bail into an emergency session where you can't mount /usr/local (which is where bash is typically installed on FreeBSD). All of a sudden, you don't have a shell.

This is a really bad place to be.