In my time managing a team that used ROS for a large European research project and my time with ROS at home, I think the hardest part of ROS is the steep curve offered by the dependency and build system - make, cmake, gcc, et al…
I offered a tutorial on ROS once to a couple of Java developers and they stopped speaking to me thinking I’d take them away from their murky maven builds and drown them in make files! 
While the existing tutorials makes it easy to setup from apt and get going, the hard part comes when you have to build topics with covariance, time synchronization and all the other good stuff to make your robot work. For example, I remember struggling to ENU transform my IMU for robot_localization and I just couldnt find a location with a good description 
even on Answers.
Still I’m a big believer in ROS and experimenting with ROS2 where many of QoS limitations are taken out. I think its the only way of prototyping robots today despite the learning curve. Building monolithic applications is so 2008 but a good option still if you want to be like Golem from the Lord of the Rings 