It can be. But it has a medium-level initial complexity so that it can scale really well to very complex systems (see caveats above).

So there's a learning curve, sure. But when you want to jump on a new system, or prototype a small algorithm on a large system, ROS will save you a lot of trouble. Also it's everywhere. Any academic or industry roboticist will know and probably use ROS, at least for prototyping.