On: Moving to FreeBSD

tcsh is the default for root on freebsd and should always be used when working as root.

why?

It's just best practice. This is what the FreeBSD-Documentation has to say about that topic:

"It is recommended that the root user's default shell remain unchanged since shells which are not included in the base distribution are installed to /usr/local/bin. In the event of a problem, the file system where /usr/local/bin is located may not be mounted. In this case, root would not have access to its default shell, preventing root from logging in and fixing the problem."

I guess there is nothing wrong with changing the default root shell to /bin/sh, but I don't know why anyone would like to do that.

(t)csh is more likely available and the default on all Unix machines, which is the root of FreeBSD.

iirc, bash must be installed everywhere else and is not as readily available except on Linux.

Posix requires a bourne shell of some sort, not (t)csh, with a binary name of sh. Freebsd provides this through Ash. BSDs have traditionally provided a C-shell, with FreeBSD being one of the few that's kept csh as the default. OpenBSD uses pdksh, and NetBSD asks.

Posix is not Unix.