One thing to remember is that DSC is very much a 1.0 product. By that I mean that its the initial release. I expect future releases to build and expand on what we have now.
In terms of scalability I don’t think the number of servers is as important as the rate of change of those servers. If you build servers to perform a particular task and then leave them to get on with that task they aren’t going to have much impacy on your DSC servers
In terms of configurations being changed by others – put them under change control! If you don’t have control of your environment it doesn’t matter what technologies you use – they will be compromised and you will get failures in your processes.