My last experience with Sourcesafe was years ago, so take this with a grain of salt. In my experience, it doesn't scale well as the number of developers touching the same code goes up.
There is no way to have multiple people working on the same code, and then merging their changes together on check-in. Instead, each developer has to lock the files they are working on, while the other developers can't make progress on anything touching those same files.
