Most likely lack of planning and applying a it should "just work" process to the scenario at hand...
The thing I've learnt with Docker is that all the other Prod issues still exist. Docker at its core only solves the executable part or distribution of the process to run. Still need to figure out, network, storage, monitoring, backups, discovery, etc...
What starts out as a single host can quickly quadruple once all the other considerations are taken into account and wanting a scalable, reliable and available system.
I've never seen this limitation with other container solutions. What is it about Docker that makes it problematic?