I havent had a single problem with Webmin. Even though this server is highly modded. (software) I run a custom kernel, scripts and modules for Webmin. And everything works like a charm.
As far as websites go theres 5 websites on the server none of them get allot of traffic. But last month before I sold my proxy the proxy website hit 248k page views and the server load pretty much stayed between .20 and .40. I was very impressed. Untill webmin is unsupported anymore its going to be my control panel of choice.
Soulwatcher