One special point of difficulty was when I wanted to install a two-cluster-nodes Content Server and wanted to use a REST API to connect third-party applications with the cluster. 

The problem was that we wanted to secure the whole thing with token sessions, and we found that their persistence is not shared between different content servers in a cluster, leading to potential problems. For example, when a process client starts a session with Content Server "A", it receives a session token that is only known by Content Server "A". If, by any chance, the next query of this client goes to Content Server "B", his token session will not be recognized and he'll receive a challenge login screen.

I worked around this issue creating a REST endpoint that managed and delivered cluster tokens instead of using those from the Content Server.