well, webmin, it it has it's nice parts and it's bad part, like most software.
Exploits do happen, and do get fixed too.
anyway, you can still use webmin for internal use, safely, by not allowing external connections to it. (internet)
(here comes the firewall to block it off)
another option would be to build a complete management script yourself for the other users, they can use.
e.g. a bash script (with ccdialog/dialog for nice windowish effects), and use it as a shell for a specific user.
As soon as that user logs in with ssh, it gets that menu, and only that menu. And can do the things you programmed it to do.
(i use such "shell" too, for some administrative things (edit configurations through reading inputs from user, etc), for co-"admins", and it works perfectly)
but this could also be done with e.g. perl, python, tcl, java, php, etc..
good luck.