The biggest hurdle moving from WHMCS I've found is the lack of a generic server module for dedicated/VPS servers.
For example, we need a way to record and display to the customer the server IP address and assigned IPs, as well as the initial server login. On the backend for a service in WHMCS, we have the following:
And on the frontend for the customer, they see the following:
I cannot find any way to replicate this using the Universal Module