I have used Webmin on multiple distros, both on the Red Hat-based side and the Debian-based side, and have had various issues with it on both. My issues have included things like configuration failing when restarting a service from within Webmin, or settings not sticking when I enter them via Webmin and apply changes. A couple of examples are with bind9 and samba. I tried to set up a new samba user but it failed to do so, and I can't even tell exactly where, I just know that restarting the server failed. Same with bind9, after configuring a zone and adding A and PTR records, the bind server failed to start and didn't give any helpful reason why, so I ended up merely using the configuration file editor to get it done. My frustration with it has led me to try other configuration tools.
It also has brought up the question -- is there a particular distro that Webmin was developed and tested on? I know Webmin is available for a variety of distros but it doesn't seem to work really well for any of them. Maybe I just haven't used it on the right one?