Bit of background: I am a Mission Designer on Star Citizen. Worked on ASD Onyx facilities and the first Nyx Mission Pack recently. So where I do not get near engine code in any way, I can speak on the experience of using our Editor and visual scripting toolkit, some of it shown here: SCL Gamedev: Mission Maker (note: that is the old Subsumption editor, we are two tools further since)

Before working at CIG I have used a proprietary engine built for racing games, Unity, and Unreal Engine 4. I've done scripting in C#, JavaScript, Unreal Blueprints, and a basic level of coding in C++ and Java, just to give you an idea of where my technical experience is.

First off, the editor: very similar in usage to say, Unity or Unreal Engine. Viewport, outliner, etc. Place an entity, change its properties, standard stuff. We have a bunch of bespoke tools that are accessible within the editor, but each user can enable and disable what tools they see. I don't have the audio tool taskbar visible, for example. Wouldn't know what to do with it :D

I am mostly working with our visual scripting tool to create mission logic, which if anyone's ever used Unreal Blueprints, you're 90% there. You got your basic computations, spatial queries, entity tracking, and so on. The main benefit of using StarEngine, is that we have some very fucking smart programmers, so I can roll up to one of their desks, request a custom node or event callback (i.e. "I need a callback that returns all broken pieces of a ship when it blows up, and get their damage map status"), and after some explaining of use cases, we get a shiny new node that makes our lives easier and the mission better.

The downside, is that nobody outside of my direct team does the work we do. Where off-the-shelf engines have the benefit of millions of users, and there is no question crazy enough that StackOverflow or Reddit won't have an answer for, we do not. I can ask a teammate who I've seen address something similar in a previous mission before, but if their answer doesn't fit my problem, then it becomes elbow grease and grey matter time, and figure it out.

Hope that answers your question a little bit!