Specifically, I use fish shell. I would really like to use this on my production server, and also set it as the default shell for the root account, but I'm wondering if there are any specific security concerns, given that it is a shell, that I should be aware of before doing so. I would guess that it is no more of a security risk than any other application on the server, since it only runs after a login occurs, but somehow using it as the default root shell on a production server feels less secure.
My main question is: is there any additional security risk in installing a non-standard shell and setting it to be the default shell for the root user than there is when installing any other application?
Edit: The current answers do not address the question, perhaps because I was unclear. Let me try to be more precise by explaining why I feel the answers are incomplete.
Of course, when you install any new software, there is a risk that it has a security bug. Certain software, depending on usage, requires more caution than other - for example, installing a web server introduces a new attack vector by it's very nature, and therefore we would be more cautious about installing some less common web browser than, say, an image conversion utility. I want to know if shells, by nature, should be scrutinized more carefully as well, and if so, why.
In addition, I would like to know if setting a non-standard shell as default for the root user would carry additional security implications. My practice thus far, even on my home machines, has been to run the fish shell as default for my own user, but not run it at all as root. Is this concern warranted, and if so, why?
So far, the answers have indicated that I should not do this, but have not, in my opinion, satisfactorially addressed the why. Practically no one uses only default software on a server, plenty of additional programs are installed on top without too much of a security concern, so why is a shell, in particular, something to be more cautious about? Ideally, I would like top see an example scenario where installing some non-standard shell as default for root would lead to a compromise that wouldn't otherwise have happened if the shell had been some other type of program (say, an image conversion program) .