I'm not a great Linux expert, but I'm comfortable running my Ubuntu desktop. I've also got a little experience running Ubuntu as a server as well, for my home network.
At work, we're considering switching away from our current Solaris based setup to something Linux based. Our sysadmin is pushing for something like RedHat Enterprise or CentOS. However, at the same time, he's pushing for the developers (like me!) to take more of a role in running the machine.
I've tried to push back by suggesting that we go with Ubuntu for the server. The majority of the development team has Ubuntu experience, as opposed to very little with RedHat. However, the sysadmin tells me that Ubuntu is not really suitable for a production server.
Our scalability needs are not that high, probably low double-digit requests per second max. For security, we need it to be "reasonably secure." In other words, we're not storing credit cards or medical records, just emails and such. Essentially, we'll be serving Ruby on Rails and PHP web apps from it.
What exactly disqualifies Ubuntu as a candidate for a production server?